I started having this thought even before rumors surfaced of the Yankees' interest in Nick Johnson.
If Johnny Damon won't come back on the Yanks' terms (and I'm of the belief that Damon won't beat a $20/2Y offer), there are a slew of free agent options who would be available on 1 year deals. Two of the most intriguing pure-DH options are Jim Thome and Nick Johnson.
Thome 2007-2009: 1572 PA .256/.380/.517
Johnson 2007-2009: 721 PA .277/.424/.410
[Update at 1:58 p.m.: Discussions with Johnson are 'hot']
There are four parts of the story here: staying healthy, making contact, working the count and driving the ball.
Clearly, Jim Thome has been better at staying healthy. Will he continue to be? While Thome hasn't been available 150+ games in 6 seasons, he's played in less than 130 since 2003 until his trade to the NL this past season. On the other hand, Johnson had hand problems when he was with the Yankees, a back problem with the Expos/Nationals, a broken cheekbone on a grounder, a broken leg after a collision on a pop-up, and a torn ligament in his wrist. Two interesting/ encouraging things about Johnson: 1) he's a year removed from his wrist injury, so he might show more power than his .410 SLG suggests; 2) only the wrist injuries are re-occurring (and separated by many seasons), and two of the injuries that have cost him the most time are freak injuries.
If they stayed equally healthy, what's the value of Johnson's extra .021 batting average? Let's imagine the Yanks' DH gets to play 130 games, with the rest going to Posada, Arod, Jeter, Tex and Swisher. That's roughly 500 PA- 128 hits for Thome, 139 hits for Johnson. While I acknowledge that a hit is better than a walk (walks only drive in runs when the bases are loaded), those 11 extra hits over a season are less than an extra hit every other week.
On the other hand, Thome would get on base 190 times. Johnson would get on 212 times. That's 11 extra hits, plus another 11 walks (which are almost, if not quite, as good as a single). At the risk of sounding too much like Crash Davis, an extra hit a week is the difference between an All-Star and a scrub.
But slugging is Thome's advantage, and in a big way. Sticking with our 500 PA assumption, Thome is going to tally 258 total bases (his best season by that measure since 2004) and Johnson will wind up at 205 total bases. Roughly, that's a 35 HR, 25 doubles, and 70 singles season from Thome, compared to a 10 HR, 25 double, and 115 singles for Johnson.
Johnson is the ideal #2 hitter, Thome would look brilliant at #5. Who would you rather have? Let me hear your thoughts, and I'll leave mine in the comments at the end of the day.