I had an ST5000 on my old boat, with the internal electric motor
drive unit, loved it. THe older processor unit had a brain fart from time to time, it was older, when I talked to Raymarine they told me I could use the same actuator unit with the new control unit, I did have to get the nameplate information off the unit to get a definite answer though. The Raymarine equipment I've had over the years was not cheap
but it was solid and dependable, except in the case of a lightning
strike, even then the autopilot survived but the knot/log meter did not.
The unit did work quite well for many years even in difficult conditions offshore
on a 38 foot racer cruiser
. The only feature I didn't find all that useful was the windvane it came with, it had a plug
in windvane unit that allowed it to steer with the wind
as well as a compass
heading, it tended to be quite sensitive and you had to dumb down the control unit quite a bit to get it to operate smoothly.
The new boat has a Robertson auto pilot that works pretty well in conjunction with the Monitor windvane but it's old and will most likely be replaced in a few years.
I haven't had a Garmin
autopilot but have had two higher end Garmin
chartplotters and wasn't all that impressed, they ran OK but the user interface wasn't at all intuitive and when they needed service
I was told they were not supported any more, I needed to buy a new one, after 6 years.
Simrad is much more user friendly and faster to boot, ease of use also counts.
Are you looking to get an all in one display unit or separate units for the electronics