Certain engines are cross-platform, others are not. In theory, the more operating systems and devices that you can get your game on, the larger your potential audience. This is one of the great advantages of an engine like Unity3D where the theory is, you write code once and run it on PCs, Macs, browsers, iPads/iPhones and even consoles like the XBox and PS3. Engines which can do this though usually charge for each export option, and each export option has its own costs associated (like the $100 developer subscription to write iPhone apps).
You want an engine which can give you as much control as you require. If you write a game in DarkBASIC, can you implement the Steam API if you want to get it on that platform?
You mention that you will be selling through a portal, in that case, you certainly want to go for the most stable engine you can find. Casual gamers aren't the most computer savvy, you'll want your game to automatically load at the right resolution for their machine and you'll need to it 'just work'. Absolutely no error messages, otherwise the portals will get frustrated forwarding support queries to you.
So out of Popcap's Framework and DarkBASIC? Definitely go for Popcap's framework, which isn't DirectX based and is way more proven.