It is quite tough to keep up with these. The two you've mentioned are actually similar in performance. The RG353m was the more powerful chip of the two but it is older and less efficient so it balances out a bit. The key difference in those two is one is made of metal and the other plastic. So you'll notice a big jump in the build quality.
Personally I prefer the cheaper plastic look as it feels more retro to me. I'd still recommend jumping in when these things are reasonably cheap at the shallow end. The RG35XX H was a proper steal for near £40. You don't want to pay £100+ for a device hoping it will play you GameCube games on the go but then realise you're only getting the performance of something half of the price.
There is however
this handy dandy Google Doc for anyone who wants a decent overview of each device.
For the most popular ones you can even find a spreadsheet breakdown of how certain specific games perform from people who have the device themselves, including the settings they used in some cases.