Hey sonic seeker. Though not always the case, 5U4G-B (straight side tube) tend to be bigger in the bottom and more "colorful" than 5U4G-ST (coke bottle shape). I prefer the STs myself, but had a long affair with GBs. Also, generally, with GBs the taller bottle ones tend to be more open, and shorter, darker/thicker sounding, with medium bottles in between. Some are more clean and some more textured...lots of variation there I think, different manufacturers and vintages sounding pretty different.
The STs sound varies also, but less-so. Some RCAs are more open and some bigger, darker, denser. Earlier does tend to be better with both STs and GBs, but not always. Some of the other ST brands are less expensive at times and can be nice too, but in the ST arena, old RCAs do tend to shine.
Some help uh!