As someone playing since closed beta and purchased some cbill mechs I've learned some harsh lessons on choosing premium mechs. If I were you I'd ask yourselves these questions before committing:
*If these purchases are inconsequential for you then just go for whatever sounds fun. The below applies if you want to make absolutely sure you're getting the most for your investment.
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Do I know I enjoy the play style?
Avoid buying something you think might be fun or looks fun but haven't tried. Ideally find a trial mech, or fit a current mech so you can try something similar first and see how you like it. Even an effective and fun looking mech may simply not mesh with the way you like to play.
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Is it the play style or the power you enjoy? Will the mech still be fun to play if it's nerfed?
Even setting aside nerfs, with the exception of maybe 5 mechs, I can guarantee that as new mechs are released and competitive tactics shift your mech will be outclassed or become obsolete. If it's purely power you're after it may be worth considering something for cbills.
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How much build variety can I get out of this?
If you already have a large garage full of mechs this can be less of an issue. But the Yen Lo Wang for example is very limited. Not just because of its hardpoints, but their placement and the mech's weight. Committing to a large ballistic takes up a huge portion of your loadout, and slots limit what you can do with the energy, etc.
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Does it compete in role with one of my favorite mechs?
Not purely a bad thing depending on how you play. But as an example, I love the Griffin. It's one of my favorite mechs of any kind, but I almost never play them. I just like my Hunchbacks that much more and always gravitate to them when I feel like an agile medium bruiser.
EDIT: Check out the quirks.
As a person above noted, these can have a huge impact. Even without Gauss Grid Iron gets a huge ballistic quirk for example.
http://snafets.de/mwo/quirks.htm
Edited by Shalune, 26 April 2015 - 11:29 AM.