In this market, if you're willing to spend some time looking you can probably buy most Marriott weeks at a discount of at least 60% and in some cases 80% off of Marriott direct prices. So unless you can get this kind of discount, buy resale but do your homework first. That said, if you're primarily buying for trading, Marriott is probably not your best choice because of the upfront cost and high maintenance fees. I think it's a lot better to buy a TS with low maintenance fees and low upfront cost if you're going to use it for trading.
If you are looking to trade into Marriotts outside of Flexchange, however, owning a Marriott gives you a big trade advantage. II has a 24 day Marriott preference, which decreases to as few as 3 days during the later parts of Flexchange, so unless you want to make trades last minute owning a Marriott is beneficial despite the higher upfront cost and MF's.
That said, you should carefully consider what your travel goals are. If you buy in a locale you'd like to visit frequently you can be assured of a nice vacation even if you can't get a trade you'd prefer. Also, be realistic in your purchase. If you need/want 2BR's during school vacation weeks and/or summer and are hoping to get beach weeks, or winter ski weeks, etc., then you will need a stronger trader than if you are content with a studio or 1BR unit and can travel slightly off-peak or even off-season. These parameters affect your purchase decision.
As for Las Vegas- the MGC is a beautiful resort but Las Vegas has a lot of timeshare units and competes with a lot of hotel promotions, so its trade value is not as good as some other areas. Buy Las Vegas if you envision yourself using it half the time, otherwise perhaps think about another locale.
Only you can decide if the price difference between you employee discount and resale prices is worth the ability to trade for points, esp. given the devaluation of the points packages. Look over this Board and read the posts carefully. There are many posts discussing this and you will see both sides of the coin, so to speak. Then, ask questions if you'd like and decide what will work best for you and your family.
There are a lot of favorite locations around here- if you offer a little more info. as to where you live and what you like vacation wise you'll receive a little more input on that question. But- keep in mind that, ultimately, you need to carefully consider what is best for you. If you haven't already, join Tug and check out the resort review area. It will give you a lot of insight to the different resorts and may be a great place to start comparison shopping.
Feel free to ask questions as you gather more info.