Arguably WBUR and WBZ are about the same amount of "all news".
WBZ Schedule = 15 hrs news/daily, 9 hours of talk (Dan Rea and Steve LeVeille)
WBUR schedule = 14 hrs news/daily (BBC, ME, ATC, H&N, MP and RB), 10 hours of talk (OP, FA, TOTN, DR and CR)
One
could make the argument that technically WBUR is more "all news" because WBZ spends a lot more time per hour on commercials than WBUR spends on underwriting, enough to add up to at least an hour's difference, I'd guess. Interestingly, WBZ is all news during the day, but all talk on the overnights...whereas WBUR is all news on the overnights, and has talk spread out across the day.
The Globe had a bit on how much WEEI's ratings spiked in just one month of having the simulcast on 93.7. I'd say that puts the whole AM vs FM issue in stark relief: if even a proven ratings draw like the Red Sox isn't enough to keep AM competitive against FM, then AM is rapidly losing effectiveness as a broad-range media. For niche products, it can still work well, but unless you've got a blowtorch signal like WBZ's, I don't think AM cuts it as a viable broad-audience transmission method.