I'm looking to break away from the usual games I play and try something new. So how's the learning curve and what Descent game would you recommend and why?

Descent is a 360° corridor shooter and is not a spacesim unless you mean (Descent:) Freespace, which is a different game.

The learningcurve is not high if you are used to first person shooters though navigating the levels can be a bit daunting at first until you are accustomed to the mapsystem which is pretty good.
Also you might want to change the default controls, I played it with the DXX Rebirth mod which worked well enough with mouse+keyboard.

What I personally didn't like about these games is that there is no way to regenerate shields (= health) and there are too few shield powerups to go by when they throw hitscan enemies at you, so you need to get killed to get full shields again.

Descent is a zero-gravity FPS, not a space simulator. Think Doom / Duke Nukem, but in full 3D. Some enemies drop shield powerups when killed, and once you get an Energy Converter, you can use that to convert excess energy to shields (but at a 2:1 conversion ratio). The converter requires you to have more than 100 energy, so you cannot use it in conjunction with an energy recharge station to get unlimited shield recharge.

Also, I highly recommend using one of the source ports instead of the original engine. Dxx-XL went for new features. Rebirth went for fixing bugs but maintaining the general spirit of the game.

As noted, this is really a 6 degree-of-freedom shooter rather than a sim. I would recommend starting with the original Descent since the levels are somewhat easier to navigate than Descent 2. I think the first two are better than Descent 3, tighter gameplay and more cohesive experience. The shield energy converter is not available in the original, but I think if you play on the easier difficulties you won't have much trouble avoiding damage for the most part. Enemies with the vulcan cannon or homing missiles are a bit more challenging, but I think the difficulty scales appropriately as you play. Dodging homing missiles is actually easy provided you have room to maneuver, you need to move toward the missile and strafe perpendicular to its flight path. Make sure you set up a control scheme that allows you to move along each axis simultaneously as this will allow you to increase your speed and I find it helps improve your aim.

If you are looking for a space sim, I'd go with Tie Fighter (I prefer the 1995 Collector's CD version with more atmospheric music using iMuse). The Freespace series is excellent as well as all the Star Wars space sims.

Ah, I gotchya. Thanks for the replies, guys.

First they have to bring it back.

https://www.gog.com/wishlist/games/bring_back_descent_1_2_and_3