External vs Submersible Pumps

The way I see it, there is nothing more cost effective than re-using as much stuff from your current setup as you possibly can.  Don't fix what isn't broken.

I believe you have already identified a key advantage to using a submersible pump; you can use a larger sump and increase the volume of your tank.  Another advantage that I see to the submersible pumps that I use is that they are easier to service since there is less plumbing involved with fittings attaching them to your sump.  It is easier to simply disconnect the output line of the pump and lift it out of the water to remove it.  If you have to install a new one, then you only have to worry about the output line of the pump and making sure that it is attached to your return hose and then you just drop it in the water.

I have heard that submersible pumps are more prone to seizing up when you service them since they are only cooled when they are working and turning them off leaves them with any heat they have generated while stopping the flow of coolant, but I have never experienced an issue with my submersible pumps seizing up, so I'm not sure how well founded that is.  External pumps, on the other hand, generally only have water in the turbine assembly and are air cooled.

For that very reason, if you have problems with your temperature getting too high, then you may want to avoid a submersible pump, because they are going to shed their heat directly into the water column.  This may also be a plus though, because if your heater is running all the time to keep your temperature up, then adding that little extra heat from the pump which is already running all the time would help you to save a few pennies on electricity for your heater.

On noise and efficiency, I don't really know anything from experience.  If I were to venture a guess, though, I would say that a submersible pump would be quieter because any noise goes through several changes in medium before it gets to your ears and therefore is muffled more, while the noisy part of an external pump is just out in the open air.  Even if the decibel levels are the same, you would probably get a lower frequency sound from a submersible pump than from an external pump, because the noise is going through the water first.

This point, along with efficiency, really depends on the specific make of the pump and how well the parts fit together.  All pumps really rely on the same principle (unless you have a dosing pump or something) of having an impeller spin around in a housing that is designed to channel the water thrown outward by centrifugal force into a single point that is the outlet of the pump.  Unless you are looking at powerheads, then this is always done at a 90 degree angle from the inlet of the pump.  I am sure that there are a few exceptions to this rule, but generally they are all built the same way.

You will often find that the external pumps are more expensive than the submersible pumps, but I think that is just because external pumps are usually intended for larger scale applications than submersible pumps.  If you need to move 3000+ gallons per hour, it is probably going to be hard to find a submersible pump that does that.