The IIIa (or III) meters are ok if the electrical connections to the photocell are intact, some wont be and will need replacing by an expert. my IIIa is still ok.
The knob wind on is slower then a Leica M lever but no worse than a Leica IIIg. There is not the rotating shutter speed dial like a IIIg or earlier, to snag and ruin a frame. The Contax shutter is however noisy and expensive to maintain... Neither camera is good for fast changing action i.e. you will only get one shot...
The take up spool can fallout if you dont invert the camera on loading, but you can select a spool that is tight (on the centre) if this is annoying, I use one from a Kiev... It is easy to load upside down anyway, a IIIg is a challenge for some.
Either sonnar f/2 or f/1.5 is superb, but a hood is desirable.
A IIIa with a lens is going to be cheaper then a IIIg with a lens, collectors have inflated the Leica. The Lieca will be cheaper to maintain as a shooter, it is a simple camera. A IIIa in ex condition is peerless on a shelf...