I own both and generally love Surefire products, however, I'd give the edge to the TLR in this comparison. Unless you can find an X400 cheaper than MSRP (often hard to do with Surefire), the TLR is too good of a bargain in comparison.
The lights on both are very bright but the Surefire does have a much brighter laser. I've found both to be rock solid (although there's no doubt the Surefire is higher quality). The TLR's activation is a little easier to use than the Surefire (the Surefire is very tight and after more than a year of use has remained very tight to activate) but I like the Surefire's momentary on which is achieved by pushing the rocker forward rather than pressing up or down.
They're both high quality lights. While the X400 sets the standard I expect from Surefire, the price is high enough that it become difficult to justify over the TLR unless you're expecting serious use out of it.