If you're happy with functions provided with ROS 6.49, then I'd say you're better off running ROS v6 for now. ROS v7 is known to have reduced speed for certain functions (e.g. routing). Having said that, I'm running ROS 7.5 on my RB951G for a while (since 7.2beta or something). It's used as AP only and it works fine. I didn't notice any difference neither in functions available nor performance so far.
If you decide to go for v7, I recommend you to perform these steps:
- create backup - both binary backup and text configuration export ... fetch the resulting files off device
- netinstall device with latest stable v7
- restore device from backup
In case you see any weird behaviour, repeat step #2, instead of step #3 configure device from scratch using text configuration export as a reminder.