Which is recommended for photographers? Not necessarily most popular but more for practicality, ease of use and functionality. I’m currently using Photoshop.
It depends on your preferred workflow and/or if you have a workflow already established. And LR is a bit like a Swiss Army knife - which is a problem if you need a jackhammer.
I use both and you'll find people that will tell you you only need the one or the other (or something entirely different). I would make the argument that there are good reasons to own both.
As far as my personal preference is concerned: I've been using PhotoShop in one way or another since the early 90s or so. I'm not saying I'm very good at it or even know everything it can do - but I know how to find what I need and learned how to use. LR can do a lot of those things as well. And the things it does it does pretty quickly and very well when it comes to editing. It has a few features that PS does NOT have but that are pretty nice. But then again there are some things that LR can't do - even pretty basic things like dodge and burn, which I like using. Yes, I know people will tell you that there are all sorts of workarounds and how it really is the only tool a "serious" photographer will ever need.
I think the main reason this is so popular with the pro folks is that it is great for batch processing and quickly going through a hundreds of photos from a shoot. It beats Bridge with that (once you've learned some rather quirky things in the user interface...).
But here is the bad part in my opinion: it is horrible when it comes to its original core function as a library management tool. It only knows one way of doing things. It's Lightroom's way or the highway. The whole concept of an image editing database doesn't fly with me. I want to have an original and an edited version. I don't care for "virtual" copies and non-destructive editing. And now with the switch from LR3 to LR4 I care even less about it because all of a sudden if you switch from one version to the next you run the risk of having all new settings applied to your entire inventory if you import everything under the new development process.
Even worse: it is extremely cumbersome to work on more than one PC with this. The new version still has no official network support and syncing and exporting databases always makes me nervous.
So, I do use it. Actually a lot more than I originally thought, since there have been more and more reasons for me to go through larger numbers of pictures at a time (event and concert shoots for example). Making selections and quick adjustments to those is pretty nice. But then I end up exporting the edits to a network folder and its backup locations - so exactly what LR really isn't meant for.
You could argue now that at that point it's pretty easy to go back later and change the editing - if you remember which PC you originally worked on...But honestly, that usually never happens. If there is a reason to than get into deeper editing on a few pictures you're much better off with PS.
There is one other important difference: LR is relatively cheap (and just got cheaper). PS is very expensive unless you have access to a student/teacher version or so. I picked up a copy of LR3 for under $100 and I have yet to see a reason to upgrade to LR4. And one more: LR (as far as I know) easily transfers between Mac and PC. With PS it's either one or the other. If you want or need both you (usually) pay twice.
Hope this helps. Again, this is my personal observation and experience with these. Some people have very strong feelings one way or the other.