These are my 2 main hotels for Melbourne stays, so have stayed many times at both, in club rooms. Neither is 100% to my liking and if I could combine elements from each it would create for me, the perfect Melbourne hotel. I now just book on price, with Sofitel usually coming out on top. For my next visit in Jan I am staying 1 night at each hotel, that is how difficult I find it to choose between them.
Mella has already commented that Grand Hyatt has smallish rooms, as you have stayed there previously you know this, the rooms at the Sofitel are a bit larger. The bathroom at GH suits me better as it has a seperate shower and tub, at the Sofitel in the Club rooms the shower is over the tub (suites have shower and tub).
I personally prefer the view from GH and the one large window which frames it, as opposed to the smaller windows in the Sofitel.
GH has a better TV set up, however Sofitel has DVD players in the room, and DVDs available from the Club Lounge.
Both beds are very comfortable, with good linen and pillow menus. I have found that noise from other rooms is more noticeable at GH.
Sofitel wins for food offerings in the club lounge, providing breakfast cooked to order, lunch, afternoon tea, evening canapes etc. GH is a bit light on, with very limited food available between breakfast and evening canapes, however I think their canapes are better than the Sofitel. Views from both club lounges are great, but probably Sofitel wins out for the stunning Yarra views.
My list goes on, but it is hard to split them. If you normally stay at GH I would certainly recommend that you try the Sofitel for yourself, you won't be disappointed.