[quote name='io']In the story mode the types are Soldier, Sniper, Defender, Scout, Heavy Gunner, and Tactics Commander. Is the one you are referring to based on the soldier-type? It seems the most generally useful in any case. I just wasn't sure what type of game the online stuff ends up being. If it is just a rush to the base then I could see a wheeled base (like the Scout) but with heavier weaponry being a good choice (if that is possible). But then maybe it is also useful to have an artillery guy in the group. I was just starting to play through the heavy gunner scenario the other day - they seem challenging but powerful. I like sniping too

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Personally, with the group I run with, we act like CAG's group and tend to fight the AI rather than other humans wherever possible. As such, since the AI tends to run various types rather than the wheeled base + shotgun combo, you would be be better off running whatever you please.
As a beginner I would second Linkin's suggestion in regards to buying a blueprint or two first since those are decent enough models to use either as they are to start or to build upon. It won't be the most effective or useful mech you could have, but it's a decent starting point. After that you can tailor it to your own needs. I used to run a Soldier with some great cannons and machine guns to draw fire/remove non-ATs from the field. The cannons were quite nice because they required a bit of sniper precision from a distance and did fantastic damage if I kept laying them into someone else's cockpit.
Also, if you have a few people on your team who have already bought all of the parts in the game, see if they will toss a few credits your way to start out with parts. Once you grab multiples of every part credits become mostly unnecessary outside of repair fees.