In general 5ms frames will give you more throughput than 2.5ms frames, but longer latency due to the longer frame size. It's up to you which is a priority. If you want more throughput, but are willing to deal with higher latency go with 5ms frames, but if you want shorter latency and are willing to sacrifice some throughput then go with 2.5ms frames.
A community member posted his real world comparison between both frame sizes in a post here, but this is from 2017 so performance may have since improved.
Another consideration is synchronization. If you make any changes to frame size on one AP it must be done to all others in order to maintain proper synchronization. This is also the case with PMP450 if you have any PMP450 in your network that you're synchronizing your ePMP with.
Just a question what is everyone running for frame size 2.5ms vs 5ms ?
which is better ?
2.5ms provides lower latency but you lose about 10% performance, and it's only available for 20/40MHz channel widths, and is really intended to be used to sync between PMP450 and PMP100/FSK systems.
5ms provides slightly higher latency (like 5-10ms more) but gives you a +10% performance boost, and it's available for all channel widths.
We typically use flexible/variable if it's in the middle of nowhere and we don't care about sync... and we use 5ms with 2.4GHz sync'd systems (as we typically use 10MHz channel widths).