For me it comes down to aesthetics and convenience.
In my eyes a one piece bow is always nicer looking with its clean flowing lines unbroken by the mate between riser and limb butt. Not that there aren't nice looking TDs, but the corresponding one piece is always nicer to my eyes.
That said, it's tough to beat the convenience of a TD. I also like the heft of a TD, one piece bows are a little lighter. If I only had one bow it would definitely be a TD. I'm in the process of thinning my herd a little, keeping my TD Blacktails, Dye, Silvertip, and Robertson...and taking the money I get and putting it toward a one piece Blacktail and one piece Silvertip recurve...