Sometimes, soloing is not about fitting as many notes as possible into the available time.
Unfortunately, many guitarists suffer from a condition I call "diarrhea of the fretboard". We don't have to stop for breath, so we can trill our picks for hours without stopping. It's actually *easier* to do this for many guitarists than it is to stop playing for a few seconds. I'm not sure why; maybe it's because we're afraid of looking bad/slow or messing up if we pause. In reality the exact opposite is true.
We can take pointers from wind instruments in this area. No matter how many notes per second you play on a trumpet, for example, at some point you will have to stop and take a breath. The same goes for any wind instrument!
Because of this, people that play these instruments sometimes have a better understanding of putting space in their solos - they are forced to put space in to take a breath.
If you've ever heard a guitarist playing "too many notes" while shredding it up, the net effect can actually be that you lose interest as a listener. This is bad, because you want your listener to be amazed at your awesome riffing!
But when you hear something awesome, and then a pause... it makes you wonder what is coming next. As a listener, you have a chance to digest what you just heard and get ready for the next tasty morsel. Check out this clip and notice the skillful use of space (remember that space isn't just silence, it means not playing as fast as you can all the time):
Eruption - EVH
Notice how long he waits at the end there after the dive bomb? That's serious space. Since soloing is closely related to speaking, check out the use of space from this great speech:
I Have a Dream - Martin Luther King, Jr.
People are hanging on every word, wondering what he's going to say next! And you have time to think about what he just said.
Using space can make you sound better. When you do play something fast (with less space), your skill is much more likely to stick out and get noticed. If you play at top speed all the time, your fast riffs are noticed less.
Love Thing - Joe Satriani
Notice the incredible restraint Satch shows here, and how good his long notes sound. When there's less space later on in the song, it sounds awesome.
Remember that sometimes soloing isn't about the notes you play - it's about the notes you *don't* play.
Use both! A skillful use of space simply means that you don't play as fast as you can all the time. Give yourself time to think about what you'll play next. That can mean letting a beautiful note sustain for ten seconds, or just not playing anything.
"It's taken me all my life to learn what not to play." -Dizzy Gillespie