And if submersible... is a Mag9.5 strong enough for a 90gal?
What is the head height? If thats your only circulation pump it would be on the low end of what I would consiter to be acceptable. But then again I like ALOT of flow.
Pumps push less water the higher the head height is from the pump to the top of your tank. So if a pump does lets say 900GPH at 0' head it might only do 700GPH at 4' head.
The specs on the mag 9.5 can be found below as far as head height loss.
The main differences between submersible and external is the following.
Submersible pumps are the easiet to install since you dont have to worry about a bulk head or extra plumbing from the sump to the pump. On the down side they are water cooled so they will transfer some heat into the water. How much this heat affects the tank is up for debate.
External pumps can usually be found to be more powerful or atleast have the potental to be more powerful than submersible pumps. These pumps are often air cooled so they do not transfer heat to the water like their submersible cousins. External pumps often require a few drops of light gun oil to be placed on the berings every X months. My little giant suggests it every 6-8 months. You have to deal with getting a bulkhead fitting and also plumbing from the sump to the pump. Its a wise idea to also install some ball valves so you can remove the pump for service/replacmetn with out having to drain the sump.