That's a good question because both coils have strengths/weakness'...
The 5.5 wins in the depth dept., bettering the smaller coil on most coins by 1-2". With larger gold rings the increase is even more impressive. The HH will bang my class ring 10" out in the air, and would likely pick it up nearly 7" deep in good ground.
A minus for the 5.5 is that it is just slightly less stable, and will belltone on deep iron more often than the 4.5... I still can generally run both coils at max sens.
Plus for the 4.5 is better target seperation in my dime/pulltab/distance test; 2.5" compared to 4" or about 31% better, pinpointing is also slightly better, making this my coil of choice for searching around public swimming pool where I use only a screwdriver to retrieve targets in order to avoid big holes and make sure that I'm not asked to never come back again!
Personally, the 5.5 stays on the 1000 99% of the time. It's an impressive little coil. I use my 1500 when I want to search clean ground and keep the 1000 on hand for trashy sites.
I would keep and use both coils if I were you, the 5.5" was on sale @ Kellyco for around 70 bucks and well worth it. Resale for your 4.5 won't be too high either and targ. sep. with this coil is awesome on the GTA, it beats the same size coil on my 1500 in both target seperation and depth for most coins, though the 1500/4.5 is hotter on gold.
Sorry for rambling... these are my observations on Garretts and small coils... hope this helps!
Skillet