As I have been watching, and learning, I have noticed that some vintage Seamasters and SM Devilles have 17 jewels and other 24. I understand the basic purpose of jewels, buy why do some have 17 and others 24? Is one better than the other, increased value, etc? And is there a way to tell without opening it how many jewels a particular watch has?

It has nothing to do with value as they're synthetic jewels.
They're like bushings, only hard so they don't wear. They also reduce friction.

The USA had an import tax for jewels and precious metal so many watch brands sent their movements to America to be put into US made cases made to their specs to save on that tariff.
Any count above 17 jewels was taxed heavily so you'll see many US caliber versions have 17 jewels (and are marked unadjusted even though they were adjusted because there was a tax on that too).
The best example is the Zenith El Primero - 31 jewels except for the American version (usually in a Movado) which had 17 jewels. The extra jewels were usually in the automatic winding system or some other extra function.

By Omega - a 'Seamaster' gold plated quartz wristwatch, brushed gold coloured dial, baton numerals, centresweep seconds, date aperture, calibre 1342, 43/643003, case 1960191/3960824, 15 jewels. Hands not operating:

All links 11/19
https://watchguy.co.uk/teardown-service-omega-bumper-28-10-ra-sc-pc-350/

https://www.wristchronology.com/vintage-ure/omega-28-10-ra-sc/