HERMS or RIMS. Benefit of HERMS is the wort doesn't come into contact with the element. disadvantage is you need alot larger vessel if you sparge as you need to have sufficient volume for your HERMS coil plus to heat your sparge water, unless you heat your sparge water in a separate vessel.
RIMS, though the element does come into contact with the wort, never sees stagnating wort across the element so you don't get scorching. I have RIMS and the only time I got scorching was when I stopped the recirculation and forgot to tuen off the element. If your electrical skills are good and you're less lazy than me it would be very easy to install measures to prevent this.
My experience of both AIO and 3-vessel is 3-vessel is better in all regards (for me at least) except space...it does take up alot of space and does require some level of faff during the mash as you have to reconfigure the hoses and attachments as you go from mash to sparge to boil...but its not as if you don't have plenty of spare/dead time during a brew so easy to keep on top of it and plan ahead. Brewdays are, basically, identical in terms of time for each system...I would disagree with the view that AIO systems give you more temp control..that is one of the reasons I moved to 3-vessel.
