Well, I'm not an expert, especially on laptop gaming, but here's my two cents.

We can't compare the MSI to the amd processor laptop because we can't compare the specs, so there's no way to know what's best. Generally speaking, Intel makes better processors, but AMD processors offer a lot of bang for the buck, so at the low/mid price range, you may be better off going with an AMD.

MSI is an okay brand from my experience. They're about standard on build quality and parts. You're not going to be brand loyal to them, but you won't hate them unless you get a lemon (which can happen with any company.)

The i5-450M is a run of the mill dual core processor. It's not Sandy Bridge, and it's definitely not overwhelming, but there are worse options. For around $500, you can find a better quad-core processor easily though, so I wouldn't waste money on older tech with the newer items are the same price.

 The onboard graphics are all terrible, despite what any salesman tells you. It can not scale to 2gigs and run like any dedicated graphics card can. That said, some games are written to work well with them (like some of the older Total War games) but many more will struggle to perform. Older games will be your friends, and your ability to play Sins is dependent on your tolerance for how low you can turn down the graphics options before hating your new computer.

Sins should run in an acceptable fashion under medium or so settings (until it mini dumps that is ) I don't have personal experience trying to play Sins on one, so that's just a guess though. The Intel HD chips are getting better, and I'm making onboard chips sound worse than they are, but they are still poor in comparison. If you want a gaming laptop, you need one with a dedicated GPU.

 If you don't have to have the mobility, definitely buy a desktop. Budget laptops have a lot of great uses, but gaming isn't one of them.