I've only recently switched from fixed to MOLLE (Santa was kind enough to bring some Bulle stuff to me :lol: ) and already I prefer the new MOLLE to the fixed.
With the MOLLE, I can changed my rig easily. I run an LSW, so I need STANAG mag pouches (and lots of mags :lol: ) - with MOLLE, should I ever change role, or weapon, I can easily. It also gives you different size pouches - I can get double- double stanag pouches (a single pouch holds 2 mags stacked together, but it has another pouch stuck on next to it)
For starters, most of the fixed ones - like the vest I used to run, and your current one (not the ChiCom AK strap-rigs) - unless specifically designed for one use, have a generic 'one size fits all' mag pouch - meaning whilst you can put STANAGS/G36/AUG/AK mags in - they never really fit properlly.
MOLLE allows you to buy pouches, bags and adons that you want/feel you need - unlike a fixed, which gives you what the designers thought you'd need. I never have (and probably never will) need 2 shotgun shell spaces.
Its up to you, but if you want to customise your gear, and have different loadouts and roles (say - you have an assault rifle loadout, and a sniper loadout, with different firearms) you can change for different games, or make a combined kit.
However - there is one main problem (imo) about MOLLE. Price. Flecktarn.co.uk has some wonderful, cheap prices for the BULLE range of MOLLE - very nice stuff. But - they are cheap in comparison to other shops. My rig was £25. But, it doesn't have anything. I spent £35 just getting the minimum (for my requirments) - my orrigional fixed vest, a highlander, which has worked well for a year, and I still like (and will keep, for 'emergencies') itself cost £35. Really is a case of cost. It also helps that I do not run a specific national armed forces unit - as I run a non descript PMC (or UKSF with civi gear), I can mix and match. If you want a specific unit (British Army, USMC, whatever) you might be more inclined to buy a specific vest/harness.
Before you buy a new rig, try your current one in a game. Or just wear it for an entire day, and see how it feels.
Personally, I prefer MOLLE. Actually, I love it (aside from the price). It works well with my PMC/Merc loadout, and keeps me in my support role. My fixed vest however, is equally useful, I just wanted a change.
Happy Christmas
(And woo! Dr Who - Amy!...I'll be quiet now...)