Most instruments are easy to play poorly and hard to play well. The thing that is most interesting is how hard they are to play competently. Many, many instruments are more difficult to play competently than either guitar or bass. This can be a function of mechanics (oboe, French horn, strings like violin, and organ (as in four ranks plus pedals) are darn hard to play) or the complexity of the music commonly written for the instrument. Strumming basic chords on a guitar or playing simple bass lines is sufficient to play a lot of popular music. Reaching only that level of competency on an oboe or violin makes you a beginner.
The skills that make for a competent guitarist are not the same as those for a competent bassist. I would rather have a guitarist with good chops and a bassist with good musical knowledge than the opposite. A guitarist lacking the ability to transition chords, mute strings, and play basic licks is very limiting. And, a bassist who doesn't understand how the melody, progression, and rhythm fit together cannot take the band where it needs to go.
I play lots of instruments including guitar and bass. If I had to boil it down to one thing regarding technique, the main difference is that getting the right strings to voice properly while muting the others is more challenging on guitar than on bass. Guitars typically have more strings, and it takes less energy to get a guitar string vibrating than a bass string. So, muting techniques are more critical and harder to master on guitar than on bass. Aside from that I think the differences are negligible.
