Well since you said you don't want to use an offset lnbf, your SNH-31 is off the list. So it's just down to the BSC621 and the BSC321SP. The BSC321SP is KU-band only, so it's obviously cheaper. From my experience, if you only want KU, then I'd go with the BSC321SP over the BSC621, as the BSC621 seems to lack a little in the KU arena (I've never got a good strong KU signal with the BSC621, as compared to a dedicated unit).
The BSC621 will work fine, but if you're not needing the C-Band side, then you might as well skip the cost and the hassle of the C/KU combo. If you need the C-Band side, then you only really have one choice to choose from given your list/requirements.