I'm certainly no expert - and I'm sure you'll receive many convincing arguments in favor of one or the other, but...
We installed both autopilot and windvane systems on our last boat before setting out across the Pacific.
Navico WP 5000 (named Otto) and Saye's Rig (named Windy).
Otto ended-up steering, like, 99% of the time mainly because it was an easy push-button operation to adjust course and it was very good at the task. Otto seemed to learn the patterns of any given sea and would end-up barely moving the wheel while staying remarkably on course... even in storm conditions.
Windy steered a much "wilder" course and would seem to loose the plot whenever we dropped into the troughs of big waves. It took a lot of "tweaking" to get her to settle into an efficient pattern of steering and any adjustments required working right at the transom.
Windy also required specialty parts to be made and / or flown to exotic places over the years wheras Otto broke only once and I was able to make a new drive sprocket at sea with tools on hand.
Your wallet has a lot to say about it, too... as one can purchase a top-of-the-line autopilot system w/ spares for considerably less than the cheapest windvane system. I purchased Otto new for less than $500. Windy cost us nearly $5000 over the long haul.
If Aye were in your deckshoes - since you already own a Monitor windvane - I would consider adding an autopilot for simple redundancy and motoring performance - either a wheel pilot or link a tiller pilot to make a "Motor Monitor"
And - don't ever try to employ BOTH systems at the same time as they'll definitely be working against each other and could quite possibly destroy the weakest link.
We traveled 25,000 using both autopilot and windvane on our last boat - a 37 ft pilothouse of 15,000 lb displacement.
With our newer boat of 49 ft & 32,000 lb displacement - I'm installing an oversized below deck hydraulic pilot in addition to a pair of used CPT wheel pilots for back-up... all at a cost far less than a Monitor system.
I believe the bottom line here is to have a reliable method of self steering while saving as much money as possible for other fun projects.