the ax55 is not a racing engine, it is more of a torque engine. the mangum 52 is more of a racing engine and loves to rev, the ax55 seems to be happiest around 13000 rpm although it will still turn props in the 12000 range decently when its broken in
imo the ax55 is one of the best os 2 stroke engines, the ax46 is decent but most of the cheaper engines transition better, are just as if not more reliable and give more power than the ax46 aka gms, magnum tt, st, ect.
the ax55 seems to be in a class of its own, the magnum .52 will keep up with it if its in a sports model turning high rpm's but it doesnt fair as well in the lower rpm range and wont turn as big of a prop without complaining. the magnum doesnt idle as well as the ax but it does transition a little better

it really depends on what plane you are trying to fly and what you want to pay, also how much time you have. the magnum will keep up with the ax turning 11x6 ish size props in a sports model, that being said you will be fighting the magnum for the first gallon, but once you have it set you never have to touch it. the ax55, if you break it in on a sports model with a 10x7 prop for the first gallon then switch to the 11x6 or 11x7 size props, the engine will give you little complaining, unless you want to fly right in that midrange lag, which in sports models is irrelevelent, but in 3d models its a real PITA.
that being said you can buy two magnum 52's for the price of one ax55 so it just depends if you have the time to breakin the magnum, or is half a day of tuning and flying the magnum worth 70 bucks to you. its your choice
i have both engines, i cant say which i prefer, the magnum is a great engine as is the ax. if i had 150 bucks and had to pick right now i dont know what i would go with, im usually a "go with the other brand besides os" guy as many will state, but both engine are so close.

btw the gms is out of stock at advantagehobby as well
that is my favorite .46 size engine of the 5 or so different brands i either own or work with including os (ax46 and .46la) magnum, tower, ect
i wont go off on how great my gms is thats for another day