I would like to know which cisco router series isr or asr is more suitable for use at branch as router with integrated vpn and firewall
Depends on size of the Branch device and network .
ISR 4000 is good, we have deployed many for medium sized branch users. make sure you buy right license for througput
here is the good reference document : ( also you need to consider Cost model)
Model can be decide once you have future visibility, what is wan bandwidth requrirement for next 3-5years. (beauty of this is, you can start with small bandwidth with Software license you can upgrade later higher throughput with out any downtime. or reboot of the device).