I agree with John - keep it.
The Speedy will always be there for you to buy later, either new or pre-owned. Don't get me wrong though - the Moonwatch is a fantastic watch (one on my wrist right now!) but these relatively obscure Heuers have a charm of their own - and I should know, because the core of my Heuer collection is made of L5100-based ones, so pretty close to this.
You'll hardly find anyone else wearing a V7750 Montreal in the wild, whatever the dial colour- so the blue one is really super-rare.
However as Jeff always reminds us, don't confuse rarity with value and don't let value drive your choice: if you like it and can afford it, keep it.
I remember seeing only another blue one ages ago, on sale from our fellow OtDer Jasper Bitter - I can't remember the value though and it was a NOS one.