With this small amount of remote sites, the needed bandwidth is more relevant. And the ASA doesn't support DMVPN until now.
In any case, I would start with a 2911 for lower bandwdths (up to 50 MBit/s; although the recommended WAN spped for that model is 35 MBit/s). If you need more throuput the 3925 is recommended for up to 100 MBit/s.
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