I am assuming the Benelli for $845 is a post-ban (I am also making the broad assumption that you want an integrated pistol grip - if this is a wrong assumption, pre- vs. post-ban is not a big deal). If it is pre-ban, definitely go for the Benelli first. If it is not a pre-ban, I would forego both from the shop and look for a pre-ban Benelli on these boards.
For carry/home def/range fun, you already have that covered with your G33. The analysis I would go through then is just a straight comparison between the 870 and the Benelli. For me, the Benelli is far superior. Another reason I would get the Benelli first is - Gaston is still making G32s, but Benelli can no longer produce "pre-ban" M1S90s.
By the way, I suggest a pre-ban based purely on my own experiences. I bought a post-ban version first. Was perfectly happy with it, but like many other gun stuff, you just have to upgrade. So, I bought a pre-ban off a 'for sale' forum, and am in the process of trading off my post-ban.
If you want a post-ban Benelli, there was a member by the handle of 'Jordan' selling one on AR15.com or GlockTalk.com for about $700 (note -I have no ties to 'Jordan' whatsoever).
Good luck with your purchase.