On the QXT I found that with it's high tones and it's recovery speed being so fast even using the 9.5" coil that the 5.3" made little or no difference in seperating targets. The QXT is that good. In fact, I found it better at seperating targets with the 9.5" than other machines were with the 5.3". The only reason I'd use the 5.3" on the QXT would be to hear smaller targets better (like chains) or to offer better audio harmonics on certain things.
For instance, I found I could better hear the double chirp caused by the two holes in a square tab when I passed over them. You can sort'a hear this with the 9.5" but it's real obvious with the smaller coil. So, it would make a good coil to dig rings while avoiding certain trash. The rings will usualy sound round, warm, and smooth, while trash will often break up or sound hollow, harsh, or tinny. This is going to be true with the 9.5" coil as well but the little coil is going to give you more audio information, or at least make it easier to hear. You'll be better able to hear the rough edges of junk targets that are not uniform in shape as well.
Now, I might consider one of those old 4" Whites coils if you can find one. They might offer even better seperation on the QXT than the 9.5".