I can't comment much on the Lincoln since I have not used that model. I've heard the Lincoln's have a harsher arc which some people prefer. Personally, I like the soft arc.
I've owned a Millermatic 251. MM 252 is the newer version of the same thing with a few extras (pre/post flow and spot timer). However, the arc quality should be the same soft arc.
Customer service/support advantage goes to Miller.
Power/duty cycle advantage goes to Miller:
Lincoln is rated for 250 amps, 26 VDC, 40% duty cycle. Miller is rated for 250 amps, 28 VDC, 40% duty cycle. The extra 2 volts is a big deal if you plan on doing lots of spray transfer or welding thick metals.
Lincoln uses potted boards which are supposed to be more reliable. I've never had any issues with Miller boards going bad so I don't see that as an advantage, but rather a disadvantage from a repair standpoint.
Warranty is 3 years for both welders. Miller is one of few companies that may honor you warranty after it has expired by a couple of months. This is not the official company policy, but I've seen it done many times.
Lincoln is supposed to have a better drive motor/feed motor assy than Miller. Again, never had any issues with 2 drive roll setup on my Millers with 15' mig guns.
Spool gun. Miller uses the 15 or 30A spoolgun. This is well built spool gun and generally preferred over Lincoln.
I think either welder will be fine and serve you well. It comes down to personal preference and what type of arc you prefer.
Couple of threads about which welder to buy:
Weld Talk Message Board and Online Forum - Hobart Welders
Look at post #9 on this thread.
http://weldingweb.com/showthread.php?t=36644&highlight=255xt
Price comparison from fleabay vendor. Miller cost is $60 more than Lincoln.
Lincoln
LINCOLN POWER MIG 255XT MIG WELDER PKG K2701-1 - eBay (item 390136612421 end time Jan-04-10 12:11:52 PST)
Miller
MILLERMATIC 252 MIG WELDER COMPLETE PACKAGE 907321 - eBay (item 360222894682 end time Jan-09-10 17:55:36 PST)