First off, he is 18 months younger than Bay. Although this won't make much of a difference for 2010 or perhaps even 2011 both players will command long term contracts. Obviously if you have to sign Holliday to a 6 year deal, but Bay is available for only 4 that changes things a bit, but I think a west coast team may dangle 5 years for Bay. Also, Jason Bay has the dreaded "old player" skill set. Essentially, folks much smarter than I am have shown that players who rely on walks and power can decline precipitously. There are always exceptions, but the declines can be massive and abrupt. David Ortiz and Travis Hafner are two examples, and writers at USS Mariner have made comparisons between Bay and Richie Sexson just before Richie signed his massive deal 4 year, $50 million deal with the Mariners. Although the parallels aren't perfect, I think Sexson's size had a lot to do with his injury issues and he was coming off a season where he only managed to play 24 games, it is a bit ominous.
Finally, despite the horrendous play that cost the Cardinals a playoff game, Matt Holliday is a much better defender than Bay. With the exception of 2006, Holliday has been an above average left fielder for his entire career and posted 3.2 UZR/150 this season. Bay, on the other hand, is viewed as a well below average outfielder by UZR (-8.7/150, a substantial improvement from 2008's -18.2/150).
To top it all off, if Bay signs with another team the Red Sox will receive a first round draft pick, as he is a type A free agent. While all the other options I've outlined in left field are also type A free agents, depending on who signs Bay the Sox could actually move up in the first round. Because of the reasons I've outlined above, if the Sox want to spend the money on a top shelf left fielder this offseason, Holliday is the obvious choice. Holliday and Bay have produced similar value with the bat, but thanks to the large discrepancy in defense Holliday has "outearned" Bay $53.4 million to $28.5 million, according to Fangraphs valuations of marginal wins.
If the Red Sox do decide that neither of the top outfielders is worth it, I believe that Bobby Abreu is a solid fall back option. Abreu is a free agent after the season, and is coming off an excellent 2009 where he played for a mere $5 million. If you've watched any Angels playoff baseball, there is no doubt this fact has been beaten into your brain already. Abreu overplayed his hand last offseason, turning down a 2 year, $18 million deal with the Rays early in the offseason before settling with the Angels. Most of the concern was due to his horrid defensive play (-25.3 UZR/150) and declining offensive skills. Although Abreu hasn't been a great defender this year a move from right field to left wouldn't hurt, especially in Fenway. Abreu's biggest problem has always been an inability to move back on a ball; with the Green Monster covering him behind, that shouldn't be nearly as much of an issue. Despite being 35, Abreu also stole 30 bases, drove in over 100 runs, nearly 100 runs scored, all to go with a 115 OPS+ . Abreu recently turned down a two year, $16 million contract extension, which was similar to the offer he refused from Tampa Bay last offseason. Should things not work out with the Angels he could be a cheap, short term solution in left field. If the Sox really think they're not going to contend until 2012 or aren't willing to pony up the years for Bay or Holliday (I don't think the money is the issue) this wouldn't be a shocking move. However, Abreu would also require giving up a first round draft pick. For a Holliday type player that seems reasonable, but I'm not sure Theo is willing to give up a draft pick for a short term player.
Another name that has been frequently mentioned is Brad Hawpe. Although the Rockies recently announced that they don't need or want to trade Hawpe, he clearly was the odd man out in the outfield rotation down the stretch and during the playoffs. Hawpe swings a very good bat (.285/.394/.519) which isn't substantially inflated by Coors Field, as he posted very similar numbers on the road (.890 OPS) and at home (.917 OPS). The problem with Hawpe is that the stat heads and the scouts agree - he's an absolute butcher in the field. He posted a -19.5 UZR/150 this season, which, while terrible, was actually a massive improvement from 2008's -46.6. While a move from right field to left field would help, as would the Green Monster, I'm not sure how Hawpe would fit into the Red Sox' plan to hang on to their upcoming talent and try and improve their below average defense. Unlike Abreu, Hawpe would require giving up prospects to acquire.
Overall, I think the Red Sox should take a run at Matt Holliday. Given his age and all around skill set I don't think a 6 year, $90 million contract is out of the question. With Jason Bay, anything beyond a 4 year deal makes me very nervous. A 2011 Red Sox team centered around a marginally effective David Ortiz, a 33 year old Jason Bay, and a 32 year old Kevin Youkilis scares the hell out of me.