One big factor you left out is the welcome bonus. The Sapphire Preferred has a 50,000 point welcome bonus and the Capital One Venture Card has a 10,000 point welcome bonus. Rumor is a better Capital One Venture Card is coming out March 7th.
I really like Ultimate Reward points for their versatility as there are so many ways to redeem them (I really like the United transfer option but I have used the statement credit option and may try Amtrak). If you would redeem them for a statement credit anyway, one advantage the Sapphire Preferred has is that you can redeem UR points for anything on your statement. The Venture card only lets you redeem for travel expenses. You would accumulate points faster on the Venture Card, but the Sapphire card gives you a bigger head start with the 50,000 point welcome bonus. I believe the Venture Card has a lower annual fee.
Only you can decide what card is better for your spending habits. But it wouldn't hurt to wait to see if a better offer comes out next week before you decide.