The AR will the cheapest to buy and shoot. It will also be the easiest to work on or modify. The HK91/G3 style rifle will be interesting and unique. That said, when a HK91/G3 style rifle needs work, it can often need the service of a gunsmith (preferably one who specializes in them). There are some things to watch out for when buying a HK91/G3 style rifle, and cjjtulsa mentioned them (though I have not heard of the PTR GI models having cracked trunions - which would concern me greatly).