I have a Miller 250dx and love it. Never have tried a comperable lincoln TIG though, but I have used some of their MIG's and have been impressed. I think the both make good machines. For me I would shop around at the local welding supply places, they will let you try the machines out, plus they will be the ones repairing the machine if it breaks. The place we got ours from said the millers almost never came in for repair, so that sealed the deal.
I also took a welding class at the local community college, and they had some miller 250s that had been rode very hard for many years, and still operated very well, so that was another reason.
have fun!
Tony