FBinNY 

I'm assuming here that you're talking about a rack with legs, and that the frame has fender eyes. If hat's not the case, stop reading this.

The carrier's legs are what carries the load, and the forward mount, whether st the post or stays, only provides stability. Mostly it's of the fore/aft type (try mounting a rack without attaching the front), but there's also an element of anti-sway. The anti-sway feature's importance depends on the rack, with some getting good rigidity from the legs alone, and others not so much. But keep in mind that most of the better racks also allow a single strap to the brake bolt, which provides no anti-sway benefit at all.

So, IMO, either mount will serve, except that on a taller frame, the clamp may be high compared to the rack deck, calling for a longer strap, and reducing rigidity. One minor benefit of using 2 seat stay clamps vs. the single post clamp is redundancy. If either clamp bolt loosens and falls out, the other will keep the rack attached and keep it from flipping backward. How much that matters depends on how you are about general maintenance, and in any case a safety strap could be improvised if you feel it's warranted.