15amp battery charger vs 4.4amp?

[Skunksmash]

I'm trying to figure out which battery charger to buy for my pontoon boat. Got 2 deep cycle batteries, in a 24v system to run my trolling motor. So here's my questions.

1. Why is one marine battery charger 15amp, and another 4.4amp? They both know that they're going to be charging deep cycle batteries, so why is one so low? Wouldn't the 15 amp charger charge them up a lot faster? The 4.4 amp charger says its fine for RV's and boats, and deep cycle batteries.

2. Do I need a 24v charger, or will a 12v charger be fine? After all this is a 24v system. They're wired in series so that of course creates a 24v system.

And here are the chargers I was talking about:

Amazon.com : Guest 2613A Charge Pro Series Marine... (15amp)

VS

Amazon.com: NOCO Genius G4 6V/12V 4.4A 4-Bank... (4.4amp)