Many folks around here got tired of tinkering with poorly functioning suction and pressure cleaners and are buying robots. Myself included! I love my robot, it way outperforms my pressure cleaner. I bought the Doheny's Discovery, more about it in my pool thread, link in sig. Also more here, First Robotic Cleaner, Doheny's Discovery
Actually Id go with a T5, or G3 but of the two you are mentioning the MX8.
I get great longevity and performance with suction side cleaners and would never consider a robot with its extra motor, and complex associated parts not to mention up front and eventual replacement price.
I've owned a Zodiac MX8 and I was happy for the first 2 years then the repairs started ...
There are a lot of moving parts and even if you are careful with line pressure etc they will begin to wear out and the expenses will mount.
I'm fine with replacing a few parts every year or few years but it seemed that once failures began, they continued to occur every 2 - 3 months.
I was buying a LOT of Side A / Side B direction control assemblies at $20 a pop, a few drive shaft assemblies at $10 a pop, and the hose was splitting at the point it joined the collar so 2 - 3 pieces were discarded due to that.
The replacement parts were genuine Zodiac MX8 from a reputable shop, if they had lasted as long (or even near) as the original parts I would have been fine.
I couldn't keep it running and that eventually became a bigger problem than the repair cost.
We double checked the pressure relief value at the skimmer ( I even bought a new one) and reviewed the pump run schedule and speeds - everything was within norms.
I bought the Dorado after I visited the shop and asked them which unit they repair the least - they felt Dorado was the one.
The repairs that they do perform mostly involve the vacuum skirt (at $15) which is a very simple part and not likely to need frequent replacement.
Everyone's experience will be different - too bad we don't have a system here where we can enter info about our models and repair history - the aggregate data would be more helpful than anecdotal stories like this ...
fla, I would be interested to know how you have your valves set up for the cleaner. I also have an MX8 with singe Pentair 3hp VSP. To get the MX8 working, I have to allow more suction thru that line vs the main drain or skimmer. That means my 3hp pump is trying to suck water thru the 1.5" suction line vs the 2.5" main drain or 2.5" skimmer line. This is very hard on pump and reduces the amount of water the pump can move. I also have to run the pump at higher RPM to get the MX8 functional.
I find this to be the biggest problem of suction cleaners; you have to run pump at high RPM, and suck more water thru the suction line as I stated.
Interested to hear what other suction owners are doing?
My setup is quite energy efficient, I run mat type cleaner (G3/T5) as I like the simplicity vs the wheeled and tracked cleaners.
I can get it going and working on 187 watts, and if I bump it to 250 I get great wall climbing and shallow end coverage and some skimming action with the T5, but I have a relatively small 20Kish uncomplicated kidney shaped pool.
Both of these units also respond well to the much higher flow when Im running solar and that means I can leave in and work and not worry about it needing an adjustment when the pump is at higher RPM to make the solar work more effectively.
My biggest problem I have with suction side cleaner is the reduced skimming.
So Ill run the cleaner a day or even a half a day or so a week then yank it out.
Polaris used to make a great product called the LCS that had its own skimmer, and with the skimmer taking some additional power I had a great working system on 350 watts that skimmed and cleaned the bottom continuously. I have lots of helicopter leaf debris influx and a few sed pods but nothing so large as to routinely clog my suction side cleaners.
All the cleaners with internal, or externally clamped storage bags (return powered polaris 360, I have friends with robots Ive tried ) let the ultra fine silt I get in my area right through.