Re: favorite t12 48\" full spectrum tubes?
What type of light do you have the chroma50s in? Is it a shoplight with a magnetic ballast? I've heard certain brands of shoplights, notably Lights of America, cause tubes to get only a fraction of their rated life. Last year I replaced the magnetic ballast in two of my older shoplights (purchased c. 1983) with new electronic ones from Home Depot. Besides eliminating 60Hz flicker, they should be easier on the tubes than the older magnetic ballasts were (although my old ballasts were apparently fairly high quality). No noticeable dimming of my chroma50s yet, although the electronic ballasts seem to make the color temp of the tubes a few hundred K warmer. This isn't a color shift due to age as the tubes appear the same color as when new on a regular magnetic ballast.
Regarding the difference between a CRI of 98 versus 91, I'm not sure if most people would be able to tell the difference. The only way to know is to try a pair side by side and see if the CRI 98 tubes appear better to you.
For both greater variety of tubes and also greater efficiency, along with the advantages of an electronic ballast, T-8 tubes are the best way to go. This is what I've been doing lately. T-12 is just antiquated technology, and as a bonus T-8 tubes require less space to store. The big three (Philips, GE, Sylvania) all make 5000K T-8 tubes with a CRI of 86. While obviously not as good as a CRI of 90 or more, these tubes are all very cheap in bulk (~ $2 or less), which makes them a good choice if you have a lot of fixtures and don't want to spend $6 and up per tube. I highly doubt the majority of people can tell the difference between a CRI of 86 and 91 anyway. For those color critical applications, at least Philips makes a T-8 tube with a CRI of 98, albeit at reduced efficiency.