Question about 5u4 tubes ?

I have been looking at 5u4 rectifier tubes. Do the older, wider "S" type tube bottles sound better then the uniform straight bottles? Are the old RCA 5u4 tubes the best of this type? If so, are these RCA's better types in both the above configurations? I guess what I'm asking is for an expert to give me some relative background on which type would be best for my preamp which uses only one of this type...but I want the best sounding one I can find. thanks, Jim