Its more 'how much do you want to spend'?
Rilfit's are nice, but pricy. Ideal knive's are relatively cheap, but take longer.
I have an ideal knife, I've done two sets of tires with it. For the once every 1-2 years I do a set, its no biggie to take longer, rather than spend a few hundred on a nicer knife. But it might be a good investment for a club.
Figure about 30 minutes per tire or so. Depends on how much you cut from each tread, how deep, etc.
tips for making it go faster:
1) Work on a hot day in the sun, if possible
2) Having a powerful halogen work light nex tto the tire as you cut can preheat the area as well.
(rubber cuts better and cools the blade less when its hot)
3) cut shallow cuts, and make a 2nd pass if you want deeper. I find 2-3 shallow passes is easier than 1 deep pass.
4) Cut slow, don't force the knife or the blade won't last too long.
5) If you don't have any more blades, slice the edge off a ulitity knife and use a small torch to heat/ bend it to the shape. It'll last much longer than the blades that come with it.