My experience with RCAs is the same. The short bottle is denser and darker, and the tall, bigger, more balanced and neutral. But both are pretty different than 5Y3GTs and also STs. I used STs first, then had a fling with GBs, and then back to STs.
Mark, as Greg suggests, if the balance of bass is good with 5Y3GTs you will very likely get notably more and deeper with both GB's, but the taller will likely have a more open midrange. This could be good or bad depending on tastes and system.
James is talking about another real option if the GB range generally suits you. For an even more big bottle sound, with a bit more body in the mids, but also a more spacious feeling, more ambient information, there is an RCA (sometimes labelled other companies) that has no bottom mica and the internals
are like STs, but they too sound different than STs. Like these:
http://www.ebay.com/itm/2-x-5u4gb-RCA-Rectifier-Tubes-Tall-Bottle-Black-Plates-3...In fact, all my pairs of STs sound
at least a little different.
I have found relatively inexpensive STs if your interest takes you there. You have to watch on Ebay, and look very closely at the construction to be sure that they not only match in tests, but also in construction...the getters shape and location, the plate shape and width, ribs or not, and the actual bottle shape and how the micas fit in the bottle. Also, they may be labelled something else. I got some really nice, true NOS Raytheon labelled RCAs for about 60 delivered....sort of like two pair of GBs.
I got a beautiful pair marked RCA recently for a little 75 delivered. Like these:
http://www.ebay.com/itm/2-tall-RCA-5U4G-black-plate-matching-pair-5U4GB-5U4GA-5U...More money, but if the ST is your flavor, really nice tubes.
I like them all and it depends a lot on what you have them with. Since you are coming from the 5Y3GT, I am thinking the best thing to do might be to find out if your preferences take you into the GBs over the 5Y3GT with that first tall bottle pair you bought, and go from there.