But I'd skip the Inspiron since you can go to their upscale Latitude and XPS.
The z580 looks dated as I read http://www.notebookcheck.net/Review-Lenovo-IdeaPad-Z580-Notebook.81415.0.html
This is also an example where the full model number matters.
I do have a Dell Inspiron i15R-2646MRB at the office. It's OK but we plan to slip in some SSD to bring it up to speed.
I know this is a very vague question, but I've always been a MacBook user, therefore buying a Windows laptop is pretty new to me. I do have a desktop computer running Windows but I believe that's a different topic for now.
I'm on a tight budget and these are what I am looking at right now:
dell latitude e6520
Lenovo Ideapad 500
Dell Inspiron 15R 5559
DELL Xps L502X
Lenovo Ideapad z580
They all have nearly identical specifications and similar reviews and prices, so I honestly don't know which to go for.