I run 3 old 19" monitors or did for a long time. Now I have a 23" in the center and that is fine but it does not play nice resolution wise with the 19" monitors. So I have to pony up for a couple more 23" guys one of these days. I was never bothered much by the cracks in between.
I am not looking at the specs on a 30", but do you not get more vertical room with that setup? If so I might take a hard look at that. If cost has anything to do with it, you might look around and see if you can pick up some cheap 19" monitors second hand and try that. If you do not like it you can crap can them and go for a 30". I will say this, today I would find it really hard to be productive with just one monitor or panel. Just too much gets going on in Cubase and related plugs along with the mix panel and other goodies to have it all fit.
Take care,
LB