Thought i'd chip in as i've been reading back through the threads for advice and this topic is kind've appropriate to me.
Personally not run a marathon in the Boston's but in a similar predicament as i'm in the process of purchasing trainers this weekend.... to break in for VLM and because I can get them more cheaply.
1st mara and personally not quite as quick as you.... 10k time in the race predictor perhaps, although don't see this as a good indicator -10k when 26.2 miles is the requirement mmm! Anyway point is, i've been looking for a balance between lightweight shoes for racing and support with the ideal for a shoe to be used in all circumstances i.e 10k races through to mara + use in training.
Just back from the shops, tried on Adidas Adizero Boston's vs Tempo's. Both fit the description in that Boston's give you more propulsion/rebound off the ground (forefoot) but Tempo's give you more support. Found them both to be equally supportive around the ankle. I'd prefer the Boston for Half-Mara distance and less but not personally for mara so think i'm gonna go with the Tempo's. Marathon expectations are 3:20-3:30 (best case) but if I was going sub-3 like you then it would be a difficult call. Think only you can decide - you'll be spending less time on your feet than me.