Having done both in the UK, at two different sites, I can compare and
contrast:
1. Auto-tow needs a hard surface, realistically at least 1000m and
preferably nearer 2000m long. Winch launch can be done off grass surfaces
needing only a route for cable tow out using a tractor or 4x4.
2. Winching only accelerates the glider (plus a small amount of engine,
transmission and drum inertia) thus reaching take-off speed takes much less
time than auto-tow where you also accelerate close to 2 tons of auto as
well as the glider (say about 0.6 tons). Typical winch launch reaches
take-off speed in 2-3 seconds whereas auto tow takes 10-15 seconds to get
airborne. This longer ground roll also exacerbates cable wear (along the
hard abrasive surface for auto-tow, compared to soft grass surfaces on
winch strips).
3. The angle of pull is more favourable as the winch is always further away
than the auto would be at any given point. There is rather more down than
forward pull with an auto, mid-climb onwards. This increases cable tension
(limited by the weak link) for a given level of 'thrust' so makes cable
breaks more likely and reduces the amount you can use back pressure to
increase the angle of attack and hence climb rate.
4. With an auto, you have to terminate the climb before the end of the
runway so you can stretch out the cable, as it falls by parachute, so it
does not end up in a big knotted pile. This reduces the effective useable
runway length.
5. Heavy cars with auto transmission generally need a lot more maintenance
than a winch (lots more effort in accelerating and decelerating the tow
vehicle itself).
My experience is that, all other things being equal, you typically get
around 50% higher for a given runway length using a winch.
Ther may be occasions, such as on expeditions, when auto-tow is expedient
but for regular training operations such as those done in Europe, you can't
beat the winch.

Regards,
Roy Pentecost