I did my PPL in a Cessna 182 (fixed gear) so got the HP part. I am guessing I might have learned to land a few hours earlier in a lighter smaller (read: a bit slower) plane. But that would just be a few hours. I don't think the HP part added that much either. But I didn't have retractable gear or fuel pumps...both of which would have taken more time and not something to make a mistake on early in training.
Part of me wished I had learned from scratch in a HP/Complex plane. Then from day one the prop, cowl flaps, electric flaps, gear and fuel pumps would be the 'norm'.
I say if you and your brother jive on all of the following then get a plane and learn in it:
1.) You both know you are 150% into aviation. It is not just a interest to one or both of you
2.) You guys get along good together. If you are always fighting or bickering or blaming this ownership thing will be terrible
3.) You can afford all of the costs. Just because you buy it doesn't mean your training costs will be significantly lower. You'll need a hangar ($100-$700/month). Stuff will break and you will both have to cough up money quickly to repair it (think $1000+ at any time). Annuals cost money and the plane is unavailable. For a complex plane maybe expect annuals of at least $2000 (hope for less). And there is that engine reserve fund. Just because you can fly it for X gallons per hour doesn't mean its cheaper. You should probably add another $30h...$45/hr towards the engine mx. So lets say you get a Bonanza and fly it for 1hr and burn 15 gallons which is about $82/hr you'll be thinking "Heck our plane is cheaper to fly than the 152 at the FBO." But you've neglected to add another $40/hr for engine rebuild bringing it to nearly $125/hr. Now you are above the 152 rental price. So in that case, why not learn in the C152 get signed off. Wait for your brother to finish. Then get rides in other planes and figure out what you really want.
4.) You'd be willing to form a official partnership. You both buy half and maybe gives each the option to sell to another individual. That could cover arguments (1,2,3 above) or if someone just lacks interest in aviation later on or can no longer afford it.
...I think your other option is to buy a decent shape 172 or Archer. Use it to learn and build hours. Use it to also learn about airplane ownership and responsibility. There will be a time when you get the real itch for speed (retracts) or grass strips or camping (tail draggers), etc. What if you want the speed and your brother wants low and slow...oops!
I wish my brother was into aviation. He loves flying along but is not interested in getting his PPL. If he did I would easily enter into a partnership with him as we have always gotten along well regarding shared interests. Then again he spends lots of time up north fishing in Canada and would want a float plane. And I would want a TTx so right there it wouldn't work so well. Then again I am lucky. My wife is also a pilot and we both wanted the 182
