I do realize this question is asked a lot and I have looked around but I find my self still struggling with it, for that I apologize.
Recently I decided to commute to work and so I bought a Giant Escape 3, this works fine for my 30 mile round trip ride (Route: http://goo.gl/VxWcHe). I work in IT and so I generally have a laptop, tablet, an assortment of chargers and other materials with me. I estimate that my load is probably around 15 lbs and is 20-25 lbs when I commute with my hobby items.
This bag just isn't working for me, it hurts when I lean forward and causes to much sweat on my back because of no ventilation. I am trying to figure out a bag that would work for my needs whether it is a backpack, messenger, or pannier. If it is a pannier it needs to be able to go onto my back when I am not commuting, and if it is a backpack/messenger I would like it to have some form of ventilation for the back. If possible I am trying to keep the price under or around $100 but I understand I may have to shill out more so I believe my limit for price is around ~$150.
Once again I am sorry for this long post, and often repeated question, but I have issues when it comes to choices (consumer wise) unless there is an obvious winner and I just can't seem to figure out a winner even with the pros and cons posted.
EDIT: Also I live on Oahu, Hawaii. If it isn't raining it is most likely very hot and very humid. Normally I am biking home while it is still 80F+ and 50%+ humidity.