Are you doing extract or all grain brewing? The consensus here is full boils generally taste better in the final product. For extract brewing full boils mean you don't have to mess around with late extract additions (adding majority of extract towards end of boil to reduce caramelization/darkening) as well.
90 min boils are preferred by some when using pilsner malts (to give extra time to drive off DMS) or if you make a big beer that requires a lot of sparging and gives you a large pre-boil volume. Then you need to boil longer to evaporate down to the appropriate volume.
A full boil does give better hops utilization (although using late extract additions in partial boils can do the same thing) and more hop oils are extracted during a longer boil. I would rather change my hop schedule or add a little bit more hops rather than spend more money on propane though.
With a full boil you do need to adjust for evaporation rate, usually something like 6.5 gallons for 5 gals of final product. Although evaporation rate depends on a lot of factors such as surface area of wort (pot size/shape), if there's air flow over the top of the pot, humidity, intensity of boil, etc. Best thing to do is do a couple of batches and keep a detailed log recording temperatures, times, and volumes at every step. Then when you get an idea of what your equipment is like, you can plug the numbers into beersmith and have it automatically adjust for your equipment.
Chilling a greater volume of boiling wort is more difficult. An ice bath is not going to cut it. You can either invest in some sort of chiller or do no chill brewing which is what I do. Read more on the newbie section and especially the thread "This vs that pro/con analysis"
*EDIT- damn beat to it and all the same answers too...
