You don't want the Opus, mainly because .50 cal paint is usually very difficult if not impossible to find at most fields/stores; it is not a common caliber at all (.68 is standard) and if you can find it, the quality and selection is poor compared to the choices of .68 cal paintballs. Kingman makes some nice markers, but the Opus is really not a good choice.
The second marker, the Sonix, is a great starter gun, but it isn't electronic. It is easier to maintain and tweak than an electronic gun, which makes it a great choice for a beginner, but if you are comfortable with cleaning and maintaining a paintgun and have a few extra bucks to spend, I'd go the next step up to the Kingman Spyder Pilot. Great electronic starter marker, it is pretty compact and light, has anti-chop eyes to keep from breaking paint, a high rate of fire, and is pretty accurate, easy to maintain, and easily upgradeable.
If you are on a budget- the Sonix is still pretty good, but without the electronics you will have to learn to "walk" your trigger fingers a bit more to get a high rate of fire, but it is still easy to maintain and upgrade, and won't embarrass you like some cheaper markers out there.