I recently went through this as well, having been out of MTB'ing for a number of years, and coming from the non-sloped top tube variety of MTB which was the only game in town in the old days (in the old days you sized a MTB alot like a road bike, and you paid for it off road).
I'm 6'0 tall with a 33" inseam and I chose an 18" Haro Mary, with a 31.8 standover and it's functionally perfect, even with the stock length seat tube.
Plenty of reach as well.
The only thing I can say is try the 'L' too..
The seat to handle bar length is critical. You need enough room there, and it's adjustment is limited to sliding the seat forward or aft.
Whereas if you go for more clearance above the top tube by choosing a smaller frame, seat height is an easy thing to solve with a longer seat tube if necessary.
Larger frames make roading easier, smaller frames are easier to flick around and handle off road. It's somewhat a compromise. And choosing the best compromise involves considering what type of riding you're most likely to do..
Therefore I would say if you do more off road type stuff, consider getting the 'L' (if there's enough seat to handbar adjustment).. If you do more on road, or cruiser type riding the 'XL' may seem like a more comfortable choice, although you may find yourself reaching too much, even with your seat slid all the way forward. And off road, you may find the 'XL' handles more like a road bike and is far too close to the jewels for comfort.
This is the best frame sizer out there, and according to your inseam measurement you need a 19" frame mountain bike. Check it out. It helped me greatly in choosing my current frame size and I can attest to its accuracy.
http://www.totalbike.com/service/frame_size.php
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