For purely visual use, as opposed to serious deep-sky photography, the LX-90 is good. It lacks a couple of things that are standard on the LX-200GPS: an electric microfocuser (you can live without it, but it makes life ever so much more pleasant for fine-tuning the focus at the high magnifications you'll use on digital images of planets) and a mirror-lock which prevents minor shifts of the image with changes in position of the scope (you can live without this one--only photographers really need it). Also, the electronics on the LXD-55 and LX-90 are a bit less convenient, since they use the Autostar rather than the Autostar II of the LX200GPS. The Autostar II has lots off nice 'hot-key' functions that keep you from having to scroll through so many layers of menus. Still, the Autostar itself is nice compared with the pre-computer days--I've used one on the ETX-90. It will find whatever you're looking for.
I love the relatively fixed height of the eyepiece for most views that one gets with the fork mount of the LX-90--this is true for planetary work. However, for pointing straight up (in altaz mode) or at the pole (if you buy a polar wedge), the eyepiece can be awkwardly close to the forks. The German equatorial mount of the LXD-55 never requires a polar wedge, but the eyepiece will end up at inconvenient heights or angles more often--you'll be stooping low at times. I don't have a feel for the quality of the LXD-55 mount. I've come to love fork mounts, but others hate them. I suspect that the lower price of the LXD-55 must mean cutbacks somewhere, but don't really know--it did get a good review in Sky & Telescope. I do recall, when I used a German equatorial mount years ago as my principal scope, that it was terribly awkward to carry. Still, if you have a garage where you can leave the scope set up on some Wheely Bars, you can use the Wheely Bars for either type of scope and be out observing in just a few minutes.
If it were me, I'd buy the LX-90 over the LXD-55, but that may only be because the LX-90 more closely resembles my beloved 10-inch LX200GPS. And since I LOVE photography, I wouldn't personally settle for less than the LX200GPS. I have, however, spent as much on accessories as on the scope itself.