Ellsbury has arguably been the most valuable player for the Red Sox this year. He has a 4.7 WAR, and is projected to finish the season at 5.5. Jackie Bradley Jr. is a promising young player, but banking on him to be more than a two win player in his first full year might be unrealistic. However, Ellsbury is going to land a massive contract, with some projecting him to land six or seven years at $20 million per year. I think he'll only get five, but that's still a huge commitment to make.
A big chunk of Ellsbury's value comes from his baserunning and defensive ability. His UZR/150 comes in at 11.3, and he's comfortably in first place in BsR, adding 8.8 runs on the bases. He's also been an above average hitting CF, posting a solid 113 wRC+. The big problem with players like Ellsbury is that speed and defense tends to peak early in careers. I think he's got Michael Bourn/Carl Crawford written all over him. Bourn has seen his UZR numbers fall way off from what they where in 2012, and we all know what happened with Crawford. They haven't been useless players this year, but Cleveland is only paying Bourn $12 million, and Crawford is still overpaid with 4 years to go.
I'd expect to see a decline in his fielding and baserunning numbers, and recent history suggests that it's probable that it could happen as soon as next year. He's going to have to hit more to make up for the loss in value. Even with his fluke 2011 season, he has a career 109 wRC+, which is above average, but it's not going to carry him when his legs start to go. Plus, with a loss in speed it's likely that his BABIP will drop to closer to the league average as he won't be able to leg out infield hits further diminishing his value.
I think betting on a player turning 30 whose value is tied to his legs is a bad investment, especially if it's anywhere near the rumored amounts that have been floating around. I'd roll the dice on Bradley Jr. and use the savings to upgrade the team elsewhere.