Both Tiahrt and Moran are conservative, but Tiahrt is more so.
I wouldn’t say that Tiahrt is more conservative than Moran—he’s a bit more stridently conservative on some social issues, but Moran is better on immigration. And while Moran has just as solidly conservative voting record as Tiahrt, his more low-key style makes him less polarizingamong Kansas Republicans, and his nomination would substantially reduce the possibility of a rift in which the RINOs vote for the Democrat (which is how Sebelius and Dennis Moore have managed to win).
I would prefer for Tiahrt to run for reelection to the House, where he can be a major player when the GOP regains its majority (if not in 2010, then in 2012).