Yeah, don't get the Nova blaster MX, if you like the Nova blaster. If the Nova Blaster is to "unforgiving" then give the MX a try. I won't ever use it, because I own face with the original. I'd rather have the damage than the RoF, because I don't spam it anyways.
Falcon is worth the coin, if you really need the help. It actually is the worst auto in the game, but combined with the SENs long range capabilities, it's straight OP on the class. It belongs on the SLD.
The Arctic SN7 is great. More damage, less clip? If you liked the SN7 at all, which has a ridiculous clip, having a higher damage version, that shoots functionally the same, is fantastic. Feels like a straight upgrade IMO, but you have to like the original first.
None of the variants are meant to be drastically different though. It's either slight damage reward for skill, or slight damage sacrifice for easier use. Check the descriptions, and base variant purchases on how you feel about the original weapon. If you suck with Arx, try the Dust Devil. If you suck with the LSF, don't even bother with the DSF. Same especially with the FM. The FMD is harder to use, and is a different velocity and arc, so it will further mess up your FM/MIRV game.
The stat differences should indicate which direction the move is in.
Crucial equipment upgrades are the x1, LAR, Spin disc (nade for JUG), the heavy shield pack, the thrust pack, Mines, and some would say the Falcon. Proxy nades are close to being on this list, but the normal SLD nades aren't bad, and the new quick fuse nades are nasty, range limited, but easier to hit with. SSF is a style choice, though the Jackal is hard to contend with. The BXT1 does it's job, but the PR may be preferred, with it's hip shot capability. It seemed to have lower damage capacity at first, but I think most people are preferring it now, though it makes the recharge pack vital. After that, it's about preference.
Completely obsolete purchases, would include the HBL (inferior to CG in every way), the twinfusors, the light sticky grenade, the sparrow, and one might argue the spinfusor/eagle combination, only because you can't pair the spinfusor with the AR. The weapons are "cross slotted" so you can't have the superior explosive weapon with the superior automatic. Until they give a weaker automatic (like the falcon) to the SLD, his spinfusor loadouts will be forever gimped, by the eagle. The only thing the SF really beats ThumpD in is range (which just makes them easier to dodge) and asset sniping (which will hopefully someday be remedied by asset driven shields. It's the worst implementation merge of the mega patch, and instead of making one class out of two, but with options, it is still basically two classes, that are now harder to switch between.