As far as which one between the two, the XM-18 might be harder to find under $500 but you can find them if you look on other forums. You might have to be patient and they don't come in lefty. Yun is great to deal with and as far as I know, he's available to take orders. I personally would get a Mini Hard or a Mini Zombie Slayer if I were to order from Yun again, but that's just a matter of my personal taste. Yun's knives are meant for hard use, just as Rick's are. Yuna knives don't have as much of a robust tip but the tips aren't delicate either. There aren't many folders out in the market today that have tips as thick as an XM-18 and some folks might see that thick tip on the XM-18 as a negative.
Just keep in mind that trying to resell any of the knives in it's lefty version will be more difficult to sell and may not demand as high of a price as the righties.