On the one hand, I don't think seedbox speed should be based on your home connection at all. It is almost entirely about how much you want to seed, and to a lesser extent what trackers you are on. The faster your seedbox can send data, the better the ratio you're going to get. If you can hit your 1:1 within a few minutes of it showing up on your seedbox, that's awesome. On the other hand, I feel that storage space is more important, so if you have to choose between the two, I'd choose space every time unless it was prohibitive. More space lets you keep torrents around longer to accumulate bonus points and catch those later leechers that weren't in on the initial spurt.
I have 150 Mbps with a Whatbox, and as mentioned that's more than enough for most applications. But I paid a bit more to get more storage space so I could keep torrents around longer. With IPT using bonus points, my speed could be zero and all I'd need to do is seed for awhile and zap it with bonus points. Now, they only count 10 at a time I believe, but a tracker like BTN counts every single torrent I seed, and while they are ratioless, you need a metric fuckton of bonus points to move up in user classes, so letting me keep dozens of those torrents around really boosts my points, even though they aren't getting much traffic at all.
On the other hand, if all I dealt with was trackers like Torrentleech, which has no bonus points at all (and assuming it wasn't fairly simple to cross-seed from IPT), I might want a quicker upload speed so I could have a better chance of sneaking in upload more often. Without upload on a site like that, you're fucked, so it would be a lot more important.
So it depends, in my mind.
And Crone, are you certain about the IPT seeding time thing? I don't remember seeing anything like that, I thought it was entirely 1:1 ratio or zap.