I'd suggest you play a round with each and see which you like better. Personally I'd go with the A-5, but it's all in how it feels in your hand.. Speed-wise the A-5 will shoot faster due to cyclone system. And for distance, all paintball markers shoot roughly the same distances. Some barrels may affect it a little bit, but generally an A-5 and a 98 custom will shoot the same distances.
As for having the tank on your back, all you need is a remote line, which works with pretty much any paintball marker. So to do this you would need your paintball marker, a remote line (get one with a slide check) and harness to carry your air tank in.