C4 was ****? That's odd...I thought 1&2 were utterly useless...never touched 3...and 4 has really been great for me. I've not been able to trip it up--full IO compensation for hardware was mandatory, and while I thought (based on past experiences w/ 1&2) that would throw off some timing--it seems to have been rock solid in that respect.
I demo'd 5 during their 30day trial...new VST Expression (reason I was looking to upgrade) didn't work for ****...crashed a handful of times in that month, where 4's crashes are VERY infrequent...and only the whole pitch/timestrech features were actually "this is better"--and since I don't use it for much audio editing anyway, I skipped. Overall, it was fine...just not really worth $200 for my cheap ass.
Anyway, the advantages I see of Reaper, other than the obvious price being a quarter of the UPGRADE price of Cubase...it's lightweight and loads stupid fast. It's built in plugs--well, they sound better on average, though C4's has a few that kill (Magneto, VintageComp, StudioEQ off the top of my head). And...well, OK that's what I see.
I like what Reaper's trying to do...but, honestly...the main function of my computer is MIDI/VI work (audio handled elsewhere)--and it's not even CLOSE to equivalent. It's audio latency was noticeably worse on the last machine, despite being so lightweight.
However...I think if all you're doing is audio production...or MAINLY audio production, Cubase is probably a stiffer learning curve and FAR more money-back to the money thing. I mean why spend what you don't have to spend? Want to multitrack audio, splice it up, and mix it? It could use some GUI in handling plug ins...but, all in all, it's a solid lightweight product for short cash. If/when I have to build a dedicated audio box, I don't have a good reason to go with anything else. The C4 license/machine can sit over in the corner and to MIDI. I'd need a second license...which I haven't even priced...but, suffice to say-that would be more money than Reaper and I'd be using all third party plugs anyway.
Why would someone who already knows their way around Cubase switch? Ehh...I don't have a good answer for that.