Here in one thread are several opinions I value highly:
Thomas Bertilsson has encouraged me to return to 35mm. Spend some time corresponding with him and he will convince you as well.
Ralph Lambrecht is right in my book... the jump in quality from 35mm to Medium Format is a significant one. Possibly all the jump you might ever need.
I'll use 4x5 because I enjoy making prints from the larger negative, but the step-up in quality is hardly visible. And Vaughn... of course you need 8x10 for Carbon printing.
On the foundation of those opinions, I am now completely neutral regarding format. It is very liberating to take any camera anywhere. Come back with the best you can get, and if you feel any room for improvement... take care of the mistakes you might have made.
And if you feel afraid to make a mistake listen to jnanian, who can help you shake off doldrums.
I have a 4x5 negative hanging to dry that I am afraid disappoints me - it is the exact same photograph as "Oak and Mistletoe, Black Diamond Mines" originally printed from a 110 negative. I took two shots, planning to provide a demonstration of the difference in quality between the worst I can do to the best I can do. Yet the branches and mistletoe in the 4x5 negative are unsharp and do not give me the delicious "fractal" effect that I had in mind. I really enjoy a black and white print which you can see details that appear infinitely fine. For the first time I understood the attraction of Red Dot Artar lenses (which I don't possess), because in this specific case, where I want extremely detailed branches, one of those lenses may have given me what I had in mind.
Fortunately for me, I take lessons like these as an opportunity to re-evaluate what is really important. And looking at the original print from 110 negative... I really like it as it is.