Just as an example, anything you see in qwerty's GoI streams can be done easily in OBS and for free. Most of XSplit's advanced functionality comes from its vastly superior plugin support. If you plan on simply adding a static overlay (or the Twitch chat via an IRC client and OBS's handy window region tool) or need to include stuff other than game footage (XSplit notably has an SWF source plugin for flash games and neater animated overlays) there's virtually no reason to buy into XSplit. XSplit definitely WON'T improve your image quality, OBS is quite a bit more efficient as far as resource usage goes. The real limiting factor is probably going to be your upload and your viewers' upload.
If you want to improve the quality a bit, you can increase the bitrate. Just remember that the higher the bitrate the more likely that someone won't be able to download the video stream fast enough to keep up. There's plenty of guides out there with their own arcane formulas for determining the perfect bitrates for different resolutions and framerates, trying to strike a balance between quality and bandwidth usage for both you and your viewers.