I prefer towable over self propelled any day and I have used both.
ASU's are infrequently used and therefore towable versions are easier to maintain and far more ractical for daily inspections etc.
As already said reversing an towable ASU can be a difficult operation but I prefer to have a chocked towable ASU with an EBT (Electric Baggage Tractor) attached than have a high revving self propelled (PTO) unattended vehicle close to the Aircraft. Or if it has seperate engine and cab is switched off there is a chance of it not starting again on completion of the Air-Start.
Oh and another thing for us mere mortals..... Towable = Cheaper
Aircraft Steps are another item I prefer towable light weight versions which can be "hand manouvered" the last few feet towards the Aircraft...
We use TLD
Driveable steps and even with banksmen the chance of damage is higher than with the towable versions...
The moeny you save purchasing Towable equipment rather than self Propelled can be invested in purchasing more diesel and/or electric baggage tractors which are the bread and butter of the ramp operation...
Hope this helps
