If you'll look at some of the old messages posted here, you'll finds lots of different preferences for a smaller coil. Some like the 8" Minelab coil and some prefer aftermarket coils made for the Explorer such as the Sunray brand, Coiltek brand, or the Excellerator coils from Kellyco. They will all work pretty well, assuming use under similar soil conditions and detector settings.
I personally really like the Excellerator coils offered through Kellyco. One in particular, the 6" DD coil, has gained a very good reputation with many users. It equals, if not exceeds, the depth you get with the Minelab 8" coil and is great at detecting coins in moderate to heavy trash areas. Bryce, a well established coin hunter here on the forum, has reported depths of over 8" on Indian head cents here, and swears by this coil. I still need to find a good site with potential to really give mine a fair trial, but preliminary hunts have gone very well. I don't think I'll miss much with this coil....
So, look through some of the old posts here...maybe do a search on 6" or 8" coils, and see what other users are reporting. Stick with a good dealer/supplier, and armed with the reports of others, take the plunge and buy one. I think its good to have a nice compliment of coils for your machine. Perhaps later, you'll be interested in also purchasing one larger than the stock coil for really deep finds!
Knipper