I would think about it like this:

Do I fly into/out of St. Maarten and the surrounding area enough to warrant the purchase.

Would I fly the Scout as actively as the other planes in my hanger, and on that same line, would I know that I would get a lot of use out of it in the coming months.

The plane you can obviously take with you across the virtual globe, while the scenery would be good if you fly into that region of the world quite a bit.

Also, how "soon" is the Duke coming out. It could be another couple of years yet.
(If I owned FSX and had a system that could support it, I would personally wait for the Duke, but that's just because I know that I would get a lot of use out of it. It fits the style of flying I enjoy doing in the sim.)

Also, back to the first two, do you fly jetliners or small props more?
Jetliners = St. Maarten and area.
Props = Scout
(arguably, although you could get both and use the Scout in St. Bart's and Saba.  )

Also, make sure you look at what else is out there that you might be missing instead of focusing on just these three options!

Some other interesting addons to consider:

Wilco's upcoming Tiltrotor
http://www.wilcopub.com/index.cfm?fuseaction=prod_tiltrotor

Other FlyTampa sceneries,
http://www.flytampa.org/

An aircraft from Carenado:
http://www.carenado.com/ecommerce/index.php3

A Dreamfleet aircraft, such as the Baron, or ATR:
http://www.dreamfleet2000.com

One of the excellent FSDreamteam sceneries:
http://www.fsdreamteam.com/

Hmmm i think the scenery would be my first choice, it really is quite stunning....surely theres some good freeware aircraft out there you could get whilst you save up for the real air ones once you have the scenery?