I can't say for sure, not being in the beta, but I don't think MWO will be quite like this. I know that is how it works in WoT, and the comparison is tempting, but there are no 'tiers' in MWO - no 'better' mechs... just 'different' ones. The features to set them apart are weapon hardpoint layouts, for example. I don't think there would be any problem with simply taking the first mech you get an playing it for the life of the game... though you might get bored and want some variety after a while
Now there will be some form of 'leveling' for your mechs, and the pilot (a representation of yourself within the game), but we don't know how that will work fully yet. In WoT as you get 'higher' it takes longer for each new level / tier... that may or may not be present here, we will have to wait and see.
Now this request is perfectly valid, and one others have expressed as well - folks who won't be able to even play right away in August and don't want to lose their premium time. The Devs have said they are looking into what they might be able to do in that regard, but we will simply have to wait and see if they come up with a solution or not. If they do, then by all means get a Founders pack - the rewards like a +25% cbill boost on your founder mech(s) will pay off over the long haul!
If they find no solution, I would still get it if I were in your shoes. Think about it this way: whether you get the boost to cbills and XP right away, or a few months down the road, it would still be the same amount (if you play about the same amount each day / month). Again, I don't expect this to be like WoT where higher tier tanks earn more credits - because, and I can't say this too emphatically, *there are no tiers!*