For what it's worth, I've used a Battery Tender Plus ($60), a Battery
Minder Plus($60), and a Xenotronic HPX-10 charger ($40) on my 9AH and
14AH SLA glider batteries. All are multi-stage chargers, and frankly,
I never saw a bit of difference between these chargers in my useage.
That useage is simple - I put the battery on charge about 9pm when I
get home from the airport, and I take it off the following morning.

My experience also agrees with Andy (GY) in a previous thread - SLA
batteries work fine, losing a little capacity every time you charge
them, until suddenly they go bad. I keep an eye on the end of flight
voltage to get a feel for capacity problems, and buy new batteries
every two years.

-John