Who knows what the ideal capacity would be, valid arguments all round. A sensible compromise might be to go for 25k but make provision via the infrastructure to cheaply add more capacity in the future should the situation demand.
That said, if the comparative cost for 35k as opposed to 25k is not dramatically disproportionate then go for the higher capacity. So there might be spare seats but these can always be sold to schools and young people at a cheap price thereby filling the stadium, creating atmosphere and developing support for the club in future years. In the meantime we would still sell out for Man Utd, Man City, Spurs, Arsenal, Liverpool, Chelsea. This would give us 6 x 15k extra gate money over present capacity, approx 90k. This equates to over 4 full houses more at our present level plus the inevitable higher gates we would get from the other games.
Yes, we would rattle about there should we get relegated. Some posters are taking it for granted that we will be. How negative is that? We have spent years realising this dream and I don't think that anyone at the Liberty has any intention of giving it up cheaply. Wigan managed to stay up for seven (?) seasons and I believe we can at least emulate them. Should we do that our club, with its sensible fiscal policies, should be set for life.