I am a multi-format user. I use 35mm for macro and holiday walkabouts, 6x6 (Hass.) for portraiture/social/landscape and most other subjects if I can persuade myself to carry the gear and 5x4 for very serious stuff - 95% reversal. If I still had darkroom facilities, and time, I would do more with the Hass. and 5x4. If you like to print BIG go for at least 6x4.5. I, personally prefer square format (better for cropping, unless they now have rotating backs on 6x4.5/6x7). Bear in mind that med./large format gear is slower to use than 35mm. Does this matter to you?
It's really down to the type of photographer you are and where your main interests lie.
I could go on, and on, and on, and on, and on, and on..........
Regards - Allan.