So another question, for argument's sake, let's say the central mirror is obstructing 20% of the aperture in the Onesky, that would effectively render it 102mm of light gathering.
No, that's not correct. In terms of light gathering, which determines the ability to see faint objects, the central mirror has a very small effect. That's because the light gathered -- or blocked -- scales as the square of the diameter. So a central obstruction of 20% actually only blocks 0.2 x 0.2 = 0.04 = 4% of the incoming light. In other words, negligible. A bigger issue in this regard is that mirrors reflect only about 90% of the incoming light. But taking all this into account, a 130-mm Newtonian still gathers about as much light as a 120-mm refractor.
Would the images be similar to those of a 102mm refractor with the same focal length? I know exit pupil would decrease, I'm mostly interested in resolving power. I've read some different resources that say that resolution would be the same, but contrast would be better in a refractor.
That's correct. The distinction between resolution is subtle but important. For observing objects with very high contrast, such as craters on the Moon or splitting double stars, aperture is all that matters. Planetary features all have low to very low inherent contrast, so any extra loss of contrast is a big deal.
A first-rate 130-mm Newtonian is roughly equivalent to a first-rate 102-mm refractor for planetary observing, but superior to the refractor for all other purposes.
And sort of an add-on question. My, albeit, limited understanding was that you could push a scope to roughtly double its aperture in mm as far as magnification goes. With that said, the 130mm onesky should be able to support ~260x magnification, a 4.7 eyepiece comes in at around ~138x. But based on results from the previous night, trying to push much harder would just be a failplan. Sure, image would get bigger, but also grainier. I would say that rather than 2:1 magnification to aperture ratio, 1:1 would be a better plan of attack. Or am I missing something?
The 2:1 rule applies only to high-contrast targets. For planets, something like 1.5:1 is a better bet, even with absolutely first-rate optics.
The OneSky has good optics -- very good for the price -- but certainly not as good as a Newt can get. More to the point, it's limited by its mechanics -- the crude focuser and decent but not great mount. In practice, I wouldn't enjoy using a OneSky much above 150X, maybe 200X in a pinch.