One quick note, the Battery Tender is "intelligent" in that it will not overcharge your battery, going into standby mode (notice the "blink" mode once battery is charged). That is the defining difference from most chargers, the other is that it is NOT a quick charger, and using that type if done "aggressively" can actually shorten the life of your battery.
I use the OEM maker of the Battery Tender - Deltran Corp in Florida, yeah it is more expensive, in fact I bought the Jet Ski / Boat version which is potted to be more rugged and water resistant as I know sometimes I may "drop" the unit or leave it outside caught in the morning dew or rain before I remember to bring it back in...not that that would happen, but "just in case"!