In my mind, the SG and the Ultra are somewhat two different bikes. The Ultra is made more for two-up comfort and for longer rides; weekends and such.
The SG is more of a crossover to me, with the front looks of a tourer, but it's more of a hot rod to me, being low slung and with no tourpak.
If you want two-up comfort with the convenience of a tourpak and a plusher ride, then either stay with the Ultra or upgrade to a new one. Heck you can always make the tourpak detachable if that's the look you're after. Me, I use the tourpak every time I go out, and the wife loves it.
If you want the lower slung look of the SG (which is undoubtedly cool) with the Milwaukee 8 engine, then get one - if the money difference makes sense to you. If you don't like the shocks, then do as Howard suggests above and switch them out.