If have reasonable light base, ~10 lbs or so, and want ONE great all around pack that will serve just about every conceivable use, get the Circuit. It will serve you summer, winter, short trips, and 10day trips, and carry a bear can well. That is why it is such a popular pack.
If the couple oz difference really makes a difference to you,(it shouldnt) you can get the Ohm. Realize the materials are lighter , more lighter nylon used on the pack sides, the mesh on back snags easily, etc. It is not as durable, that is a tradeoff. My Ohm has a couple of places where the thin black nylon has abraded "almost" thru (can see spot of light thru material) from hard objects in the pack or side pockets.
Honestly, with the stretchy back pocket I can fit as much in my Ohm as in my Circuit. But the Circuit carries it better.
If you have to ask if a frameless pack is right for you, it isnt.
Any pack can be made frameless by removing the frame as well. You can remove the foam and stays from both the Ohm and the Circuit to save a couple oz if you want. What you will find, is that they are worth the weight. It makes packing easier, and more comfortable.
The only reason to have a smaller, lighter , pack is that you just want to shave 1 lb. Dont stuff your sleeping bag, Dont put gear in outside pockets, and either the Ohm or Circuit is suitable for any load, no matter how small.