RTM3K and RTA4K are exaclty the same software, same board, same components, same memory, only less memory is enabled on RTM. I would be surprised if you find a unit were typical noise is diferent, but they must exist, because RS put it as datasheet spec, noise in my RTM is within RTA spec. Oscilator on RTM can be calibrated to match RTA. With RTA you get quite better probes and maybe a full calibration certificate instead of a 2 page one. If you need bandwith, get bandwith, all the software features don't really make up for that. Any external AWG is better than the build in, which is not that flat, and has no internal or external sync.