In my experience,Keyboards are easier to explain/understand, when it comes to theory (that is why colleges teach theory on the keyboard) ... as manxcat has pointed out, pianos are linear.  However where the guitar really shines, is in learning scales.  On Piano (or a lot of other instruments) you have to learn different fingering to be able to play in diffrent keys (12 major scales, 12 minor scales and that doesn't even take Modes into account).  On Guitar you only have to move the same pattern up or down the neck to play those same scales.  Learning one fingering pattern allowed me to play in most keys.   This really makes it easier to transpose into other keys (a very useful skill for playing with others).  If y want to play the same lead line or song in another key, y just move that fingering pattern up or down the neck... presto chango you are there!  It may take more work to ingrain that scale pattern into your brain and fingers on guitar.  But once done, playing lead or even playing chords in diffrent keys, gets much easier.  For me, that made theory a bit less complicated to  understand and use.

I think your Piano experience can be very useful if you can move your practice habits from the keyboard to the Guitar.  Learning persitant prctice habits is usually the hardest hurdle for Guitar novices.  They have been playing the instrument for a couple of months, then they come to a difficult skill (like barr chords) and they hit a brick wall. They get frustrated that they still don't sound like their heros, and they skid to a stop.   If you have played piano for most of your life, you have probably developed practice habits that allow you to keep putting one foot ahead of the other, without getting bored/ furstrated.  Here on this forum, you will hear, again and again, "Learning Guitar is a distance race, not a sprint".  Most beinners just skip over that wisdom and fall flat on their face as a result.

I would guess this is the greatest reason that most novices give up playing guitar.  As you learn Guitar, make a point to latch on to things that you enjoy playing.  That could be riffs, or rthyms, or songs, or what ever excites you.  When you hit difficulties learning new stuff, be sure to spend practice time going back to polish/ develop those old favorites.  This can be extremly helpful when you hit the inevetable brick walls of developing new skills.  When that happens go back to back to the songs/ riffs you love.  Have some fun with them. 

I also find that I end up getting much better at those old riffs/ songs as I incorprate new skills into those old favorites. Those favorites are more fun than just banging your head against a brick wall.  As a side benifit, those old favorites will also grow with your developing skill set. 

Having fun is the greatest reason to play guitar.   If you stop having fun.. you will likely stop playing guitar.

 Most of the people who suceed at learning guitar are people who have learned how to enjoy practicing.  They have fun at it instead of getting flustered by it.  As I said, you probably already have this skill, as you have been playing piano for most of your life.  That practice skill will be the key to growing as a guitarist.  It places you well ahead of most novice Guitarists here on GT.  Enjoy the flight....