IMO, the IIIa is - by far - the better buy for the following reasons:
It has slow speeds plus a higher top speed of 1/1000.
It has a lens which, while not the best by a long shot, will serve until you can purchase another. You may be surprised and decide the Summar suits your style.
Chrome wears better than black enamel even if it's not as "cool" looking.
The IIIa has been recently CLA'd which means it should be in better operating condition.
Now, having said all that, here's what I'd do. I'd email the seller and inquire about the rangefinder accuracy and contrast. I'd ask about the condition of the shutter curtains and if all speeds work as they should. I'd ask if he knows of any faults that don't show in the pictures or haven't been disclosed. Finally, I'd ask if he'll guarantee that the camera is functional and as described.
If you're satisfied with the answers then by all means become the proud owner of a Leica IIIa. 🙂
Walker