i have both and since i don't shoot in raw i have never had the problem...
so if you plan on shooting in raw a lot it might matter to you, i've never seen any difference in jpg .
not sure how many pics they are talking about but in continuous, i think the most i have taken are maybe 5 or 6 at once but haven't had a problem with that in jpg
i also have a converter so i can use sd cards in my rebel and that seems about the same as the cf, maybe a little faster...the cards i use with that are 1 130 card( san disk something or other) and 2 60 cards( microcenter own brand) and can't say i see a ton of difference with those either, at least not to the point where i missed a shot i wanted due to the speed of the card
i don't use flash much but that is the only time i remember having to wait for the camera and that doesn't have to do with the card, that was due to the flash
steve'sdigicams had this to say about prospeed cards ( and it leaves op and me out in the cold

" To get the most out of a cards like these your camera needs to have an optimized CF interface. Digital SLRs such as Canon's EOS 1D Mark IIN, 1Ds Mark IIN, 5D, 30D and 20D, Nikon's D2X, D2H, D1X, D200, D70 are all well suited for these high performance memory cards."