Depends on what you want. I have the xperia, while my wife has the omnia. The omnia is a nicer size for me, with a big screen but lower resolution. The xperia has a keyboard (which I thought would be important but I find that I use it less and less). Her omnia came with garmin navigation which is nice. The xperia has a 3.5mm headset jack so you can use most earphones without any adapters. It also uses a standard mini-usb charger, so there is no lack of chargers if you are away from home.
In the end I think it boils down to personal preference.
Apart from monitors, I'm not a fan of Samsung in general. A mate of mine had an Omnia that I found to be quite sluggish compared to my ancient P3400. I've never tried an X1 myself but another friend of mine has one and is most happy with it.
I was going to get an X1 but decided to get a Touch Pro 2 instead because of the case cracking problem on the X1.
I get the impression SE has seen to this problem as there don't seem to be as many complains lately. Can read more here: http://www.xperiax1.net/forum/xperia-x1/1137-your-x1-cracked-22.html
eish most phones are pretty damn good nowadays... i have x1 and thrilled with it, since december last year. what keeps it fun for me is the new ROMs and software released every week, never get bored with phone.