I'd take Eastmond over Tomkins every day of the week.
The amount Tomkins is overrated is crazy, he's a handy enough player but not international quality. The media harp on about his offloading (like they did with Banahan) but fail to recognise that just because of you attempt a ton of offloads doesn't mean they're all great - how many of these go to the opposition, to ground or into touch? He's like Banahan, in that regards - he seeks contact then throws the ball out hoping it goes to one of his team mates.
I can't see Eastmond playing 12 for England, unfortunately. As you said, I think Lancaster wants someone bigger at 12, which is a shame. Could do a job on the wing, but then that would be playing him out of position, as he's focusing on 12 at Bath at the moment, which wouldn't be ideal.