I have an MXT, and all three of the coils you mentioned. I use the 4x6 the least, and the 5.3 the most.
In my estimation, the 6x10 and 5.3 are roughly equal in depth, coverage, and accuracy of VDI and depth readouts. It is really a toss-up between the two in situations such as you describe.
The little 4x6 is really a specailty coil, and I only use mine in EXTREME trash, such as an area around a picnic table that's littered with hundreds of pulltabs and shards of cans. I recently got a Classic ID, and this detector gets that action now! It tunes out the trash very well.
Since you already have the 6x10, just use it for now. When you get into the garbage, turn down the gain a click and go veeery slow. Later on, in a year or so, cash in the clad and put it towards a used coil. Anyway, that's how I got all my coils.
There has been alot of debate about the Excellerator coils being the best choice. I have not used these, but they might be worth looking at. Personally, I am happy with my Eclipse selection.
I wouldn't sweat what you don't have, and enjoy the coil you do own. It is probably the most versatile of the three.