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.