I just commented on your other post about the booster seat, but I also own the space saver and love that too. Depends on how you plan to use it, but I'm glad we have both. If I could only have one, I'd do the space saver, b/c that is what we use most of the time, that set up works best for our family.
I liked the space saver b/c it did recline pretty far back, so I could let/teach DD to hold & drink from her own bottle as a fairly small baby still. We don't pull this chair up the dining room table though, it stays in the kitchen where I normally feed her dinner on her own as soon as we get home from DC. Its a good height that I can sit on a chair & feed or watch her while she eats. About it sitting up straight - I never noticed that as a problem. She leans forward on her own to eat (I don't strap on the shoulder straps anymore, she sits great in the seat and doesn't try to climb out), so never seemed to be an issue. We use the booster seat at the dining room table when we do eat together as a family on weekends.