Here is how I would use the Stream 3, my apologies to whoever I lifted this image from, I just got it from Google images. This set up will create a gyre flow clockwise and then counterclockwise as you alternate the pumps.
To me these are very different pumps and the reason to choose one over the other depends on needs.
6150 is fairly large which is a disadvantage, probably the only one. It is very heavy and solidly made, it really is built to last almost forever only needing fairly inexpensive spare parts over the years which would be a big plus. It is also relatively noiseless 42db maximum and it could be quieter but we had no quieter surroundings to test in and what noise it does make is very low frequency so nearly inaudible. The flow is very broad and gentle and largely fills the tank, it also tends to round corners nicely. I have found that the high flow through the pump with the levitation and the backwards rotation makes them very low maintenance, in nearly 6 months I had no noticeable calcium build up, just some algae and bacteria slime and a bit of sand dust.
The 6105 is more versatile, smaller, easier to aim. It is a conventional DC motor with hall sensors and TF capacitors, giving it an expected life span of almost ten years, not more, the limiting factor is the capacitors, after approx 7-8 years they no longer reliably store a charge, hall sensors can also be complicated as a major overheat can shift them out of alignment with the rotor. The flow is far more directional, which neither good or bad, just a difference that may be better in some applications but not all. The noise is 52 db which is still relatively quiet, you will generally find just a room with the AC blower on high and hard flooring will have about this ambient noise from air coming in the registers and escaping under doors. They need cleaning every 3-6 months in a typical reef tank set up to remove calcium deposits. One main advantage is bigger magnet holders are available, the 6105 can with stronger magnets be used on up to 1" glass, for 6150, the strongest magnets available only work for up to 3/4".
