I got both the 1-12 Little Armory P90 and the Orange Cat P90 (from ebay). They're both the same size (their magazines are the same dimensions too), and have the same high level of detail.
The only thing I can tell that's different between the guns is the detailing for the bolts/fasteners holding both sides of it together are stylized a bit differently for each (circular depressions on the LA version, recessed circles on the OC version. Just different artistic approaches).
The LA magazines has nice bullet detailing (a little bit of brass paint will give a nice impression of a loaded clear plastic magazine), and you get two; the OC magazine has no bullet detailing, so it looks empty- and you only get one.
The OC P90 does come with two rails for attachments, and the gunsight (I think there's a new LA release of various scopes and accessories- a p90 sight/scope might be in there).
They both have a supressor attachment for the barrel.
There's another LA P90 release with a long barrel, btw (I don't have that one). If you get both the LA and OC version, then you could give the extra mag to the OC and the extra rail to the LA. If I had to choose one, I'd get the LA version for the 2 more detailed magazines.