There really are no "best" systems when it comes to flat roof since every system has its' place.
http://reliableamerican.us/articles/...questions.html Kind of short compared to my usual write-ups but there is some basic advice on choosing a low slope roof system.
To elaborate, what's more important that the roof type to be installed is the roofer installing it. Everyone has their favorites. I have a few favorites that depend on the situation and there are pros and cons to each.
I can tell you to be weary, very weary of the $500 price. That just doesn't seem like enough. 3 squares of materials for a 10 year modified bitumen roof are approximately $250, plus shingles for tie in maybe another $75. How much are you going to pay the workers? $5 an hour? C'mon. Be very careful with this contracotr, if he is even a legitimate contractor, and exercise your due dilligence with extreme caution.
Furthermore, more information about a torch down roof system and an IB roof system is needed. For example torch down can be installed to a 10 year spec or a 15 year spec or a 20 year spec? which one is it? IB, although I am not very familair, I believe has the same system ratings 10,15,20... what changes is the manner in which it is installed. A 10 year modified bitumen is a base sheet and a smooth cap sheet while a 20 year modified bitumen is a double base sheet, smooth mid ply and granulated cap. There's just too much gray area to compare apples to apples.
80 mil PVC single ply roof membrane will last a lifetime. The wood you just built will rott before the roof fails, if properly installed. I can say that while I do not install IB, I do install PVC and if I were to roof this porch with an 80 mill pvc membrane I would be in the same ball park as the other roofer, probaby slightly less but I don't know enough so am really just guessing.
I really like single ply roofing systems like PVC, but going back to my previous comments about every roof having its' place, I often opt for a color coordinated granulated modified bitumen roof system when it's visible on residential and especially when it ties in with a shingle roof. There is nothing wrong with a shingle to pvc tie in, but the granulated modified bitumen looks more natural when tieing in with shingles. You may consider a torch applied or a self adhering. CertainTeed makes a flintastic and a flintastic SA. Remember, how it's installed determines its' longevity.
http://reliableamerican.us/articles/...contractor.htm some basic advice on choosing a contractor for your roofing project(s).