Personally, as someone who has owned both, I'd go with neither - they're both crappy monitors really. Try and find a 17" VGA CRT from a PC, and a VGA adaptor...you should be able to find a newish one pretty cheaply, and it'll probably be a lot better than either the ColourPlus or the MS15".

Another option, if you're happy being stuck at 640x480 is the Macintosh Colour Display. Its a small 14" Trinitron that was often bundled with LCs...should be fairly easy to find, and despite being fixed at 680x480, they're a very nice little monitor. The Macintosh 16" Colour Display is good too, but takes up a bit more space than the 14" MCD.