Re: What speakers have better sound? 4-ohm 16-ohm
This is an enormous question to answer in one mouthful.
To start with the simplest part: Celestion and Eminence are regarded as the two chief companies making speakers – think of them as parallels to Seymour Duncan and DiMarzio. When it comes to aftermarket speakers, Greenback 25 watt speakers and Vintage 30 speakers are probably the two most common choices from Celestion; with Eminence I don't really know, because they don't seem to get much use in Europe. There are also a number of smaller boutique makers, and I am sure other people will have their recommendations.
For you, I think it would probably be the easiest to get a speaker that is known to work with what you play. If you ever choose to change them, you will have a frame of reference, ie what you want them to do differently. Since I don't know what type of music you are playing* or what you want to sound like, I can't give any specific recommendations. For my own use I love the top end of Greenbacks, but if I were playing a seven-string their bass response would probably have been too weak to be optimal. You might have other preferences. This is another reason why YouTube videos are of limited use: they are usually going for a completely different tone than I am, and guessing what it would sound like in a completely different setting can be different.
In a way it comes back to the first thing I said: there is so much gear out there that is at least good for the right application, that it is impossible to give a definitive opinion of what is better and not. For now I can really only give you two pieces of advice: (1) let us know what kind of tone you want out of it, and what you will be playing with it, and I am sure there are people here who can give an educated opinion of what should do the job well; (2) even better, try out different speakers if you have the opportunity to do so. No advice we give here, and no comparison on YouTube, is nearly as helpful as trying out a piece of gear for yourself.
*: You nick gives a hint, of course, but I am not qualified to give opinions on grunge gear, as it is a sound I am not intimately familiar with.