Okay I will be the guy to jump in and say that an extra 5 feet can be a pain depending on where you camp. We have a 34' 5er bunkhouse and just bought another 34' 5er rear living room. We cant go over that length due to a short driveway. In addition several local campgrounds are a tight fit already. If we were full timing and knew there were good pull-thru sites everywhere we went then 40 foot rigs are plentiful and we LOVE them! Just consider if a 39' rig will fit where you want to camp...and if it will then the extra towing length will be easily overcome with some practice as mentioned by others. The most important thing is does the floorplan work for your lifestyle, and if so then will it fit where you camp. After that I think the decision is made for you.
good luck with whatever you chose.
Daryll