On both, I think the ideal would be a pair of 6255's and a 6215 wavebox. Placement will largely depend on tank layout but either centered on either end or in a back corner, in about 6-8" from the side wall and aimed diagonally across the tank will generally be best. The wavebox would have to go on a side wall aimed down the length of the tank.
The biggest issue I see with the 6105's is that such a big tank likely has 1" glass or acrylic and it will cost $100 to get the magnet holders for that thickness of material. I would either sell them or keep them as emergency back ups should a pump got down in the future.
The 300 is at the lower end of where I would use 6255's, so you could very feasibly use a 3rd and possibly 4th 6105. The wavebox and 2 6105's could even be enough depending on the type of SPS and how heavily stocked and filled with rock the tank is.
How thick is the glass? In general I prefer multiple pumps as it gives you a greater safety margin if one goes down. Keep in mind that out of the box a 6105 is only suitable for up to 1/2" glass and extending this to 3/4" adds about $50. The 6255 out of the box works on 1". 2 6255 is cheaper and occupies less space, but 4 6105 gives more flow sources, that means you can aim at more potential dead zones and as I said, if you lose a pump and it needs to be repaired, it is much less of an issue. Both could work, I think 2 6105 and 2 6255 are overkill. Really, 2 6255 is likely overkill but within reason.