Ok..so this is annoying me: The Inc is the successor of the Eris, not the predecessor.
As for your question, there are two ways to look at it. One is that you obviously need a new phone relatively soon, if not immediately. So why not run your Eris until death and see what is out then? There may be something better, there may not be. Option two is I WANT A NEW PHONE NOW!!! syndrome. Many of us here suffer from it. In that case, go for the dinc. I cannot predict what will come out or change price when, but I can tell you the following:
HTC will use a Snapdragon. The newer snapdragons are marginally faster (it is a phone...you may not notice the jump from 1.113 (OC DINC) to 1.3GHz). Heck, the jump from 1 to 1.113 is even barely noticeable...only on the things that lagged ever so slightly (not as bad as that Eris dialer). The newer snapdragons are also 45nm, so slightly more power efficient. The point is, this is not the speed jump of eris to dinc. The processor technology jump that you would get from waiting is minimal.
8GB internal storage may be made larger, it may not be. SD card slots work fine for most applications.
The Sceen is not getting better. It may get bigger, but probably not better. This is a personal decision and a gamble as to whether anything does change.
If you want an HTC keyboard, then gamble and wait.
After all that is taken into consideration, I say the DINC is at best the best choice and at worst a good but not optimal choice. In hindsight, I think I would take the DINC as I think the size is about perfect. Go hold one, find a friend with one, or just stand in front of the store version and play. See how bad lag is. Install a graphics intensive game trial or something. Then decide. In the interest of fairness, do the same for the X or Galaxy S.
-Nkk