I have a GB SE(2.9) now, and had an X5. Depth wise, I would say the X5 is probably a little deeper, but that is with the stock 9 inch concentric coil on it. With the 5x10 DD on the X5, the units are probably very similar ( GB with 11 inch DD). Recovery speed, the GB is a bit faster. X5 hits silver a bit better. Low conductors I would say even but I do not have any experience relic hunting so I don't know how targets like bullets, mini's, etc. react to these units. I am using gold jewelry and nickels as the comparing targets and the DD as the coil.
I do know the GB is much more stable in the wet, salt sand I hunt than the X5. I always had to tone the X5 down and there were a few beaches were certain 'hot rocks' would drive it to the point where I could not hunt with it no matter what I did. Dry sand no problem.
I think the advantages the GB has are its visual display and 2 tone break point...the main advantage the X5 has is its build quality.
The GB ground balances easier and much better in wet, salt sand and can still run at full gain on most beaches I hunt.
There are some great deals every so often on used X5's, but not sure what happens if something needed to be fixed on one. Seems they rarely need fixing since they are built with high quality components. A new GB comes with a 5 year warranty.
My 2 cents. Sorry I added a little more input than just a depth comparison !