Hugh Robjohns wrote:Why do you think it needs upgrading? What is it not doing, or no longer doing, that you need?
It's really about sound quality of the recording - can I improve on it in any way, superior new tech compared to stuff from a few years back etc.
I don't really have any idea if say a modern Apollo Twin will be superior to the RME if I record two clean guitar parts side by side, will there be a difference sonically
Short answer, no.
A little longer: a Twin gives you access to buy and use UAD plugins. Also, the Twin has Unison preamps, which apparently allow you to change some of the hardware characteristics (e.g. impedance) via software. This is not used directly but by the UAD "preamp" plugins, which doing so can better emulate the originals they want to emulate.
I say "apparently" because, even having a Twin, I never felt the need of using it that way. Of course it can be because I have a couple of the actual hardware of their more popular preamp emulations ;-)
Beyond that, so long your gain structure is sensible, you won't notice much difference between preamps and A/D conversion in the two. The converters may not sound exactly the same but both companies produce top notch designs and in a mix (or even side2side) it's very unlikely you would hear any difference.
If anything, look at improving your recording room or get better microphones.