1st time buyer - 31 vs 35 ft trailer?

The length difference between 31' vs 35' is not an issue as far as towing. With 31' trailers you will find a small advantage over the ease when backing into sites. You will find more opportunities to use State Parks in Ca. and some of the older smaller RV parks. I think that Ca. state parks have a limit of 30'. The real important issue on selecting any trailer is rather or not it fits your family size needs. You should also consider what type of RVing do you plan of doing i.e. how many rv trips per year, just week ends, two week trips, 2 month trips etc etc. Part of answer also depends on your RV locations and weather. Areas that have a lot of rain or has real hot temps or cold will mean that you will spend more time inside the RV, thus comfort and space can become more of an issue. The size of your holding tank is more important to me than the length. the bigger the better and easier RVing living will be generally speaking.
In general, most of us when picking out a trailer/5th wheel don't really have a realistic picture of the size difference between living in a regular house and a 5th. wheel.