After adding color last year I think I am set for the next few years. I also use a RasterOps MediaTime card on my IIx, also a 24bit color card. I've found with an LC that only programs that run real slow or use graphics tons are that much slower at 8-bit. Games are the only things that have been real significant for me, with sound also being a factor. I'd suggest you go for the colour - the difference is amazing.
Cost is a factor, but going for something with at least the option for later is a real good idea.
Just an opinion. I don't know where this myth came from, but they are certainly untrue from my experience. In any case, let me attach a summary I made up of my experiences in choosing a color setup last November. Let me know if you have any further questions. Before I begin, I would like to thank all of the people who helped me make my decision last November, especially Meric Ozcan who introduced me to the Lapis card.