In all honesty I think you will find the food on Seabourn to be exceptional, based on what I have heard. I have seen pictures and it honestly looks incredible. Silversea has excellent food, but it has a heavy European continental accent - which not everyone finds appealing.
The ships and itineraries are very similar.
I think the suites on Silversea may be more appealing - they are big and very comfortable. The televisions are nicely hidden away behind mirrors - very high tech.
On both ships all of the staterooms are mid-ships and forward - which seems odd to me since forward is the roughest part of the ship. I prefer a stateroom in midships or aft myself.
Especially considering these are smaller (relatively) ships they can get a little rough if you are not used to the motion. For that reason I would avoid the open Atlantic in autumn.