Hi Simon, here's my view, but am no expert on these very early guys I'm afraid. Still my 2c for what it's worth:
1. 2446c. Keep the original Tachy, first because you'd have a different one from the MH you already own, second because it's the way it was born. And you'll have a two very definite watches, a dress one and a racing one.

2. 11630 GMT: it is very lovely on the strap, I'd keep it as is because I love this kind of small-hole straps, they do seem to complement Heuers so well and so specifically I dare say. Also, this is a chunky beast and most end pieces end up bent early on - no such risk wit the strap. Not to mention the fact that bracelets for these - as for most Heuers alas - have reached outrageous prices lately.

Compliments for a wondeful batch of Autavias!
Cheers,
Fabrizio