Bronson Arroyo was the Red Sox third best starter in 2004, so you have to include him in the comparison between the two.
If I had to break down the rotations between the two it would look like:
Number one starter
2004 Schilling vs. 2007 Matsuzaka
Schilling is no longer the ace of the rotation. The best pitcher is clearly Matsuzaka. Anyway, in 2004, Schilling posted a 150 ERA+, which was good enough for second in the AL. I absolutely loved what I saw from Dice-K against KC, and I think he'll be a top five starter in the AL, but he won't match what Schilling did.
Edge: Slight 2004 Schilling
Number two starter
2004 Martinez vs. 2007 Beckett
Pedro slumped badly from his 2003 form in 2004. He posted a 125 ERA+, a 3.90 ERA, and a 1.17 WHIP, which is pretty damn good by anyone's standard. I expect Beckett to match these numbers. He's shown he's bettered his curve and changeup, and is learning to mix them in. I've got Beckett posting an ERA slightly below 4.00. There's no significant difference between the two.
Edge: Even
Number three starter
2004 Arroyo vs. 2007 Schilling
Bronson Arroyo was very good in 2004; he posted a 121 ERA+, which was five points higher than Schilling's 116 ERA+ in '06. I think everyone expects Schilling to regress a little bit in 2007, and he confirmed it, when he said he's pitching to contact. (See: more hits, higher WHIP, less K's) If Schilling puts up an ERA around 4.30, I'll be pleased.
Edge: 2004 Arroyo
Number four starter
2004 Wakefield vs. 2007 Wakefield
Wakefield was exactly average in 2004, posting a 100 ERA+. He hasn't really moved off that number the last three years, and I don't expect him to change that any time soon. He may be the most consistent Red Sox starter every year. I'm tempted to make the 2004 Wakefield the favorite, because his battery mate didn't suck nearly as bad as he does now, but that isn't fair.
Edge: Even
Number five starter
2004 Lowe vs. 2007 Tavarez
Derek Lowe was terrible in 2004, posting a 90 ERA+, why they didn't punt him out of the rotation is beyond me. Tavarez was surprisingly competent in 2006, posting a 103 ERA+, but you can't be average if your WHIP is 1.56. The advantage of the 2007 Red Sox is that they won't allow Tavarez to suck as long as they allowed Lowe to. Jon Lester will replace Tavarez in July, and will represent a huge upgrade.
Edge: Even for the first two months, Lester the next three.
So, I have it at:
2004 Red Sox - 2
2007 Red Sox - 1/2