Come on, dude. Seriously. It's time to take off your training wheels and learn to ride a bike. Next year you can earn your Harley and do 10 gallon batches AG style!
Ok, now for reelz. Just do what you want! If you really like the MB fermenter, then that's fine. But, most recipes are for 5 gallons and fermenter buckets are very cheap too. Then you have the option to ferment a 5 gallon OR a 2.5 gallon batch. Headspace is not an issue for the primary, except you might actually want MORE headspace to allow for the krausen.
I vote for the bucket, but not because it makes you look cool (that would be a conical...), but because it is more diverse. I brew half batches sometimes and primary in a bucket (I usually skip the secondary, but if I really wanted to secondary something like a big beer I can rack into a small carboy for that purpose). The primary fermentation will push the O2 out of the container, so no worries.
So consider if you want to brew larger batches in the future. The bucket will be ready for you. MB, not so much.