Honestly, most of the guns are pretty much the same net result imho. You'll find the main differences are trade offs between rate of fire versus bullet damage. You'll never find a "true upgrade" to both stats at the same time.
Your best bet in my opinion is invest in the utility/armor certs first which are general purpose to your preferred class before investing in the weapons themselves (aside from maybe a scope or something if you like fighting more at range than close quarters, etc). Being able to take a few extra hits or hold an extra grenade or having your shield recharge faster is more important than having a bullet hit harder less frequently in my opinion. For example, as a combat medic, I upgraded my medtool and nanohealing ability exclusively. The gun is kind of there just in case.
That, and the weapon unlocks cost a lot more (in terms of cert points) AND you have to rebuy any grip/rail upgrades for the new weapons if you had previously bought them for your old weapon - they do not transfer.
Plus, as was said, you can trial the weapons at your leisure to see if there's one you prefer.