7.62x39 penetrates a bit better, has the ballistics of an artillery shell past 200 yards.
5.45 shoots flatter, tumbles earlier in tissue, save for a few select 7.62 rounds that performed similarly. Can't find massive cases of ridiculously cheap surplus ammo anymore which was an original point of purchase. Days of 12 cents a round surplus ammo are forever gone. Hard to find quality bullets for hunting/self defense as the majority is steel cased imports.
With cost and availability at it's current state, I'd recommend the age old duo of AK47 or AR. Mid-level Ar's currently cost about the same as mid grade AK's, and the only difference from a low-mid grade ak and a 2000 dollar one are fit and finish really. If you want a flatter shooting accurate cartridge, 5.56 will outperform 5.45 in the given cost range of rifles and ammunition with easier acquisition.
If Tula still manufactures 122-124 grain Hollowpoints with Uly scored bullets, these are fantastic. I grabbed a case or 2 when I sourced it a few years back.