I've got all the coils you mention and use them all depending on the hunting conditions. Here's a quick rundown on my observations and opinions.
4X6 - great for very trashy areas and for getting close to playground poles, metal fences etc. Must hunt slowly and methodically as the coverage is small and while not a depth demon, deep enough for coins to 5-6" . Fun coil to use.
5.3 - good in trashy areas, better coverage than the 4X6 and surprisingly good depth for its size. Actually is a very good coil to leave on for much of your detecting. Monte loves it.
6X10 - good ground coverage, decent depth and good target separation for its size, the Double D configuration is a plus in higher minetalization. Also a good coil to leave on for a good share of your detecting and my personal favorite.
9.5" - Good coverage, very good depth, but in some of the trashy parks I hunt, it doesn't separate between targets as well as I prefer.
12" - plenty of coverage, very good depth, but again isn't all that great in trashy areas.
Over the years, i've found that I use and get generally better results for coin shooting and general detecting purposes using the standard and smaller coils. When I first started the "bigger is better/deeper" notion prompted me to purchase several larger coils, but ultimately except for hunting in pretty clean areas or situations where that extra inch or two of depth may really matter, they spend more time in the closet or vehicle than on the detector. All this is my personal opinion and observations, others may differ.
Hope this is helpful.
BB