Careful with gun size, a gun you may want to buy, based on looks may end up feeling useless to you where you play.
Don't get me wrong, you can work around your guns length but eventually you'll want something a little more easy to deal with.
Starting small and going up is less expensive then going big then moving down. For the simple matter depending on where you end up playing, you may never feel the need to move up. Such as at UA where full length battle rifles are just not worth carrying around.