[Subzero]

Hi,
For whats it's worth,we have both 501 and 393 and this is our opinion of them for the use you have suggested.
501 = Solid and well made, heavy.Ideal for digiscoping and we have used it with a 500mm (prime) lens with 1.4x/2x TC and heavy pro DSLRs.We found it ideal for STATIC type shots of birds ie in branches/trees but very slow for birds in flight.We found that we could not pan fast enought for fast flying birds, crows and gulls no problems, anything faster we found the damping effect of the head was too "sticky".We think that this is not a fault of the head but more that we were trying to use it for something it was not really designed for.The other concern we had was that we could not balance our kit, ie if we tightened it then we could not just grab it and pan or tilt without loosen the various locking/tightening controls.I hasten to add that we used the standard plate and that the elongated plate MAY have given better balance.
393 = This is, as already stated a gimbal style head, cheap and works as is, ie you don't need extra plates for each different big lens you own, and is about 1/3 to 1/4 of the price of the other big two.It consists in very simple terms of two "U" brackets.The big one fits on your leg set for the panning action; and the other bracket "hangs" within the outer brkt by two adjustable turn screws to give tilt.The tilt bracket also has has 3 height settings ( your choice).
If you are into birds in flight you will not find better for the money, not only is it very quick with (in our case 500mm lens +TCs+Pro dslr), but much more importantly in our case, PERFECT balance ie you can leave the set up as you last left it, once you have initially set up your balance point for your camera/lens combo.For the money, £100, you won't find anything better!There is a v.good/fuller review on ? (Andy Bright/Birdforum??)The only slight problem you may encounter is that that the LH side pieces of the brackets MAY impede your access to various controls of your lens, or of a particular make/size of lens.We are going to modify ours, by cutting off 45mm ish from the inner bracket (LH side only), and then use the centre set of holes for the pivot point.We believe this will give much greater access to the LH side of the lens (where most of the controls are).We have also made an "L" shaped ally flash bracket 200mm long x 70mm , (cost about £3) which utilises the two spare holes on the RH side of the inner bracket. This allows the flash + flash extender to point/follow the same direction/angle that you point the lens.
Hope some of this is of use.
Regards Subzero.