DVD+R burned with a DVD-ROM booktype approaches 100% compatability with the players available. However, in order to do a burn like this you need a burner that supports changing the booktype and burning software (like Nero) that supports setting the booktype on +R media.
That being said, there are so many other factors which influence whether or not your DVD will be playable, it's not simply a matter of +R vs -R (I'd stay away from RW media entirely for something like this).
Quality of your media can have an enormous impact; buy cheap media and get less than pro looking results. Your burner can have an impact - some burners are simply better at burning than others. Burn speed can have an impact and slower is not necessarily better. Encoding bitrates - too high can be just as bad as too low, as can allowing too large a difference between high and low (if VBR).
If the festival has a specific format that they want, then I'd burn for that format. Another thing you might want to do (if your really worried about it) is to take a look at Nero's analysis tool - it can be very useful looking at burned media to see what the overall burn quality was.
Let me jump in here and recommend "Taio Yuden" DVD-R 8x blanks (www.supermediastore.com or www.meritline.com). Only about $0.30 in quantities of 100, and they are the best-performing disks I've ever used, based on bit-error tests with Nero's test software. You can do a search on this board for more information--this has been discussed many times.
As for bitrate, you go as high as you can to eliminate any MPEG2 artifacting, but the limit is about 9.5Mb/sec for both video and audio combined. Some people on this board strongly suggest going no higher than 8 for the video bitrate. This will fit on a DVD-5 disk as long as you have less than about 1 hour of material.