I've had a Marmot Hydrogen, Helium & Lithium.
I've had a WM HighLite, Badger, Kodiak SMF & Bighorn SMF.
I currently have a WM MityLite, Sycamore MF, Sequoia SDL & KodiakSMF.
At 6'2" & 200lbs I'm mostly legs so I like the hip/foot girth of the WM semi-rectangulars (named after trees). The Sycamore doesn't have a collar but whichever jacket I carry makes a nice collar and will keep me comfortable to about 15dF if I need it to.
Some folks are good with narrow bags but I like to have room to move. Plus as I get warmer I can shed clothes and they stay warm in the bag with me. With the big bags I can also get shoes and water bottles inside to keep them from freezing. Though the big bags weigh more and use more pack space it's a trade off that's worth it for me.
When you buy a WM bag you're paying for craftsmanship and the best materials available. If you're Hydrogen is a long model the girths are 64"/60"/40". Look at the WM spec chart compare to see where you want it bigger for the temp range you need.
...ask for the old paths, where is the good way, and walk therein... (Jeremiah)