DK et al are right on. The BOSS is the best for three reasons: affordable; versatile; and they have a model that will fit your style/situation.
For example, I like mine because: you can have it in line with your guitar cable; and, it has both lights and a meter, so you can get things working perfectly.
Now, if you need to get back in tune really quickly but not super-accurately, look at the small Sabine unit that sticks to your guitar body (but does not damage the guitar surface at all.) Small and basic, but I've seen them on stage a lot. I use one when needed. Otherwise, use a BOSS.
As for a Petersen strobe tuner, in my opinion, every tech should have one on their workbench, but it is an expensive unit for a player, and not really needed, except for intonation and tech work.
On an aside, the tuner question also depends on your situation. For example, if you are playing blues or hard rock (SRV to Deep Purple), you'll end up tuning to the organ (probably a Hammond B-3).
Meanwhile, if you are into metal or another situation, being able to get yourself in gear perfectly is nice (a la Neil Young...each member of the group uses the exact same tuner and model...see 'Rust Never Sleeps.'
Finally, as a guy who was around before the personal electronic guitar tuner was invented (I've been playing since the 50's), don't get dependent on the tuner.
Learning to tune your guitar without a tuner is great practice in itself and will help you out, like when you have to quickly switch to an alternative tuning between songs, etc. In brief, use it, but don't abuse it.
Hope this helps.