You will get many opinions on this subject. I have had both submersibles and now externals, so I can speak about my experience with both.
You don't mention how long your pipe runs are, how big your pipes will be, your filtration, etc to give you a good figure on what kinda pump you'd ned. But just based on the info you've provided, I would not get something smaller than 4000-4500 gph.
Submersible:
Positives:
1. Submersibles are great because you just toss 'em in the water and plug it in. Very simple and easy.
2. Great for smaller ponds
3. Can be cheaper, but once they push large gph, they tend to mirror prices of externals.
4. Quiet
Negatives:
1. Have to get your hands wet to maintain them, and potentially receive an electric shock. Uncommon, but it happens. It's often not easy fishing a submersible full of water outta the pond, particularly from an underground pond. It was a back-breaker for me, as the cage was all full of water.
2. Clogs with debris inside pond and need to be frequently cleaned. Putting them up on bricks so they don't sit directly on the pond floor can help here.
3. usually not as efficient from a energy consumption perspective as external
4. If it gets clogged, the pump can float up and begin dumping water outta the pond. It happened to me several times, and left me with inches of water and nearly lost the fish had I not caught it in time. One time I was on vacation, which was not pleasant. A few leaves had fallen into the pond and gotten sucked around the cage, causing it to float up. Had I not had the pump on a brick, it would have certainly left the pond dry. (so, if going with a submersible, DO NOT put it on the pond floor). However, one time it tipped over and the fish were flopping on the pond floor. So make sure you secure your pump.
External:
Positive:
1. Sits outside the pond, so it's dry hands maintenance. No need to get in there and fetch it outta the water. You can clearly see what is happening with the pump visually at all times.
2. Very energy efficient and can deal with head more effectively. Good for larger ponds of 2,000+ gallons
3. Quiet
4. Has a leaf trap in front of it so debris is stopped before getting to the pump and grinding it to smithereens and sending it to the filter/pump. This fact also protects the pump from damage.
5. Can deal with head more effectively. Long pipe runs, very high waterfalls, etc. Externals are made to push water, which submersibles are good at sucking water.
Negatives:
1. Can be more expensive upfront, but you usually make up for that with lower wattage use pretty quickly. A larger submersible such as you would need, will likely be very close in cost however.
2. This is not typically plug and play like a submersible. You need to think about piping and how it will lay out. The pump will also need to be below water line so that it can receive flooded suction and be gravity fed. [I don't really consider this a negative, but I guess someone who was intimidated by this fact would).
3. If the pump isn't below waterline, you have to deal with priming it. And it's just not the most effective use of the pump. [Again, I don't consider this a negative, because a priming pot/leaf trap in front solves this with a check valve in the water.]
4. Typically has to be protected from direct weather. Needs to be covered with something, but still receive air circulation. Also, put it on a cinderblock to prevent it from getting flooded in a rain storm.
Also, in regard to an external and the fear folks have about either popping a hole in the side of their liner or using a bottom drain (weather retro or standard), if you can gravity feed a barrel or stock tank--the external can feed directly from that. The pump does not know where water line is. That is, if waterline is the pond or a barrel.
I'm sure others will give you different opinions from people that have had experiences with both externals and submersibles as well. For me, having used both of these types of pumps, the external wins hands down. It's just been way easier for me to maintain, has been gentler on my electric bill, and they have more power to push through pipe runs, waterfalls, filters, UV's, etc and the whole mangle of filtration. I certainly could not have a sumbersible doing what my external does. And I haven't had to deal with a tipped over, clogged pump again.
That's just one opinion. I'm sure you'll get others.
