I've had examples of both for years now.
Lethality is probably pretty similar between these two, but 7.62 undoubtedly has better barrier penetration. The 5.45 has a reputation for being nasty in soft tissue, and I have verified that the bullet does indeed tumble wildly when shot into a variety of mediums.
The 7.62x39mm cartridge is much more common and easy to find, and bullet selection is much better. The rifles and magazines are everywhere. It's a fun-to-shoot round, but the weapon will bounce around a bit in your hands and it's not nearly as controllable or accurate as the 5.45.
5.45 has become much more common than it was 5 years ago, but it is still a bit of an obscure/exotic cartridge in this country. Surplus ammo is easy to find and bargain priced right now, but soft point or hunting ammo is scarce to nonexistent. Also, this cartridge is not currently domestically manufactured, which means you have to rely on imported ammo for now. The guns and mags are very common right now, though some have been of dubious quality over the years. Stick to one with an imported (i.e. non-American) barrel and stay away from the Lancaster builds and you should be fine, though. If you get a 74, I would reccomend you get a large supply of ammo for it now as well.
My personal preference is the 74. It's more accurate (2 to 2.5 MOA), particularly at distance, lighter recoiling, more controllable, and generally more fun to shoot to me. The ladies will certainly like the 5.45 better, as with a 74 muzzle brake recoil is pretty much nonexistent.
I suggest you go handle and shoot both and then make your decision. Or just get one of each
