Hi Tim, and welcome. I live nowhere near either (I'm near Portsmouth), but as luck would have it, I went to Bletchley (getting off for the first time) on Monday 13th July, and was quite pleased with the photographic potential. Six platforms: 1 & 2 used by fast non-stops, 3 & 4 used by most other traffic, including freight, platform 5 (didn't see it being used) and platform 6 on the east side of the station, used by the Bedford trains. The Bedford line curves directly east from the station itself, going beneath a bridge & passing the TMD on the other side, so that's NOT very good for photos, but if you're on platforms 2/3, you're in position for anything using the up/down fast and the up/down local. The two groups of tracks are seperated at the south end, meeting up again just after the road bridge just past the end of the station. If you'd like to see what I managed to do (photographically speaking) at Bletchley, there's a good few photos in this collection:
http://daverowland.fotopic.net/c1725838.html
Hope this is of some use to you. Have fun, Dave.