Either of them would be a good starter AR.
You may hear more negatives about the Oly's than most other brands out there, based on past problems. Keep in mind that most of those issues have all been resolved with the current models. But almost every manufacturer out there has had some teething problems with their products at one point or another.
Now, as far as keeping value goes, I would probably say the Stag would be easier to sell down the road if you had to. Simply because there are so many out there who read and believe everything they see on the internet, and so they tend to shy away from the Oly. But also because Stag is made by CMT (Continental Machine Tool) who make alot of the AR parts and pieces for alot of the industry, not to mention the Military and Government.
I have had Stags, and they are very well made pieces, reliable and accurate. I can't say that I have owned any Oly AR's, but I have handled them and shot them. They also seem to be very well made, and went bang every time I pulled the trigger. And they never fail to get my attention every time I see one of their ads, where they advertize calibers like .243, .260, .300, etc. Something that very few offer.
But then again, once you get your first AR, you'll find how easy it is to just pull two pins, swap your upper assembly, magazine, and ammo, and be shooting anything from .204 to .50 caliber...from the same lower receiver.
So, take a look at both of the ones you mentioned again, either will work well for you as long as you take care of them. Most of the time, you'll find it call comes down to what symbol (roll mark) or brand name you want on the side of your receiver, what furniture or other options you want on it, what you want to feed it, what color you want it...all things that are easily changed, too. Any questions on "how to", you've come to the right place.
As for which one to buy...your choice. They both work.