Im currently looking at moving up from a 1996 26 scarab, to a 31 1996 twin scarab, how do you guys feel about the 31 as it compares to a 29 with twin 350s?

Pretty much night and day. The 31 will be/is significantly heavier, has a wider beam, most likely an enclosed head, and best of all, comes with big blocks.
Given the choice, you can see which way I would go

would i 31 be a better ride in rough water cause the 29 being so narrow i think is awesome

You are talking about 2-3000 lbs more plus another 3.5 feet of hull length. The 29 runs good in head seas and reasonable in following seas. I do not like the way they run in quartering seas. This is strictly one mans opinion, whatever thats worth

I had your same dilemma and I ended up upgraded to a 96 31 last fall and and don't regret the decision! Scarabman is right. The 31 is longer, has more room below deck, enclosed head and best of all big blocks. The down side is, the 31 with big blocks will use more fuel and you need a larger vehicle to tow it but I still think it's worth it.

My close friend had a 29 with twin 350's and when he would run with the big guys, he would use half the fuel. That is an obvious plus but whatever your decision is, they are both great boats.

I have a Scarab II with blown big blocks and TRS drives and I am very happy with a boat with a narrow beam as long as it is moving. The fuel cost is low, it performs very well and is easy for one person to handle docking and loading on and off a trailer. If I were to change I would jump to at least 38 feet or more.