What you have doesn't require allot of flow, but I think I would still go with the 2 Seio 1500's. The Tunze pumps are great, top of the line, and a fair wack better than Seio, but they come with that price tag that reflects that. For your money you would get better flow out of the 1500's.
Now if the comparison was between the 2 Seio 1500's and 2 Tunze 6100's on a multicontroller it would be a different story
no chance of the 2 6100's and controller unless I hit the lottery !
Should I put 1 Seio higher up in the tank on one side and the other one lower near the bottom on the other side (I plan to mount using magnets)
or should I try to put them on the same level so they currents meet in the middle of the tank ?
The only thing I can say is put them both in there and play with it. I have a 1500 on the right side of my tank, and a 820 and 620 on the left. These pumps are aimed so that the streams of flow meet each other on the front pane of glass in the centre of the tank (50 gallon).
This works for me, but you might not like it much.