Mold 1 of Circle Upper is found exclusively in early Takara releases of MA-04 as well as Random Booster ACT 2's Death Gargoyle (Red).

Mold 1 is significantly more likely to wobble in Stamina combos, this does not noticeably affect performance.

Mold 1's Metal Frame ramp is roughly a quarter thinner (2.9mm) than Mold 2's (3.6mm), this does not noticeably affect Attack performance.

Mold 1 is more likely to be in the high 19g to low 20g weight range.

Mold 2 is round in later releases of Takara's MA-04, Random Booster 3's Death Gargoyle (Purple, Light Blue), Samurai Changer MS, and Hasbro releases of MA-04.

Mold 2 is not prone to wobble nearly as much as Mold 1, this does not noticeably affect Stamina performance.

Mold 2 is much more likely to be high 20g to low 21g in weight (21-22g if using Samurai Upper's plastic frame).

The initial design of Metal Upper (Mold 1), manufactured in Japan, features less metal reinforcement, and a more blunt-edged ramp and Tiger's head.

In general, the later Metal Upper release (Mold 2), made in China, is more common and more resilient than the Japanese first mold. The Metal Frame's edges in general are also less blunted, with a more curved shape to reduce stress but not significantly detract from power.

The added reinforcement via thicker metal on the second mold's bridge is a welcome addition and also helps Metal Upper's weight a touch. You will notice on Mold 1 that the piece of metal securing the outer part of the ramp is significantly thinner than in the second mold. 

The top of the tiger's head is also reinforced in Mold 2, with metal connecting the circle component to the triangular horn. In the first mold there is no such bridge, leaving the tiger's head more susceptible to chipping or breaking over time.

This means Mold 1 is more frail and prone to breaking, and considering that Metal Upper is preferred in right-spin, the ramp-edge is the primary big contact point and this version's is blunted, a thin reinforcement bridge is begging to fracture. Mold 2 is definitely the safer choice.

Similar to Metal Upper,  but likely most similar to Plastic's Driger V2, is the slight mold adjustment found with Dranzer MS' Spiral Upper.  This minor tweak was made to improve durability more than anything else.

Early Spiral Upper's (Mold 1) have a slightly less thick Metal Frame design (3.1mm at the edge) compared to later models (Mold 2). This difference in thickness is about a millimetre and barely noticeable unless you pay very close attention.

There is no significant difference in weight, nor is there any significant change in performance between the two.  

If you must use Spiral Upper, it is technically ideal to use the second, slightly thicker, mold. This is purely to make sure your AR choice is as durable as possible over the course of hundreds of battles. If you only have a Mold 1 Spiral Upper, it won't be significantly inferior to Mold 2, just technically less likely to last quite as long. 

No noticeable or consistent weight differences despite using a different Plastic Frame. 

Advance Balancer's Plastic Frame actually manages to cover some of the gear-like metal protrusions, perhaps making it slightly better for spin-stealing.

Weight distribution is more spread out and even on Advance Balancer's Plastic Frame.

Samurai Upper is consistently a gram or two heavier than Circle Upper, along with a significantly larger and more prominent Plastic Frame.

Samurai Upper's Kabuto makes a minor appearance as a contact point extending slightly beyond the Metal Frame, whereas no part of Circle Upper's Plastic Frame extends beyond the Metal Frame.

It is argued that Samurai Upper's larger and more robust plastic bolsters its attack power while Circle Upper's conservative plastic lends itself better to Stamina or Defense combos. If there is a difference, it is so minute it cannot be clearly and consistently observed. 

The two should be seen as interchangeable in terms of performance, with preference being whether you personally think 1g of weight up or down would make a positive difference for your specific combo at that time.

All of these ARs have roughly the same weight range despite wildly different Plastic Frames, with Devil Crusher being edging the other two out by just a bit.

Upper Dragon and Upper Fox's Plastic Frames do not greatly hinder their attack power, Upper Dragon in particular is fairly good while Upper Fox lags behind in left-spin. In general these two ARs are middle of the pack in Attack combos, which are their only usable combo type.

Devil Crusher's Plastic Frame makes it noticeably worse than the other two for Attack, which are more or less dead even. The recoil generated by Devil Crusher also makes it not particularly good for either Stamina or Defense. This is definitely the worst of the trio.

In terms of weight, Aero Knight edges out Turtle Crusher by a bit thanks to its thicker and larger plastic, but this isn't particularly impactful. Both have wildly different contact points because of their distinct Plastic Frames.

Knight Crusher is noticeably less powerful, its metal contact points consistency obstructed by the Plastic Frame to the point it feels like a totally different (and worse) AR than Turtle Crusher. It has far too much recoil and not enough Smash Attack.

Turtle Crusher is a deceptively powerful AR, that when hit in the right angle, can send many things flying out of the stadium. Unfortunately this is not nearly consistent enough to warrant use in a competitive setting. Nevertheless, don't count it out if you come across an Attack combo using it, all it takes is one good hit.

Wildly different Plastic Frame Designs, yet all of them are well below my informal 20g "viability threshold". 

Smash Phoenix had the potential to be a decent Stamina or Defense AR, but its performance is muddled by its serious recoil and it does not have enough weight distributed across the AR to make it worth using anyway.

God Smasher is unfortunately not suited for any particular type in HMS, it doesn't perform well in any viable setup, only serving as a hindrance to the other parts. This is because it shares the same recoil as the other ARs with this Metal Frame, but no real upsides. The Plastic Frame gets in the way too much to generate meaningful Smash, and it isn't conservative enough to be less recoil-prone.

Smash Leopard, despite being the first release of this Metal Frame, is also the most viable of the three. It makes for a quite decent Attack AR with recoil that can be tempered to some degree by fast rubber RCs like the Grip Flat Cores. In play it only performs a hair short of something like Metal Upper, with the potential to be a top end Attack AR.