There are several attributes to consider when you buy a study bible.

Of course, a lot of it has to do with personal style, readability, and comfort. But I thought a more objective list might prove useful:

• Leather-bound: nothing else will last long enough. If you do serious bible study a hard back book will last a few months, a paperback version maybe a year, a bonded leather version maybe two years, and a top quality genuine leather bound version (with smythe sewing, if possible) will last at least a decade. I still have my first Harper Study Bible that I bought back in the early 1980s. Some of the pages are so heavily marked up that daily use is almost impossible. Several of the pages started to fall out in the late 1990s, and I took it out of my general use about then. However, it is still an indispensible  resource for my writing and I use it daily then (though very carefully as it is an old, tired friend).
• Many cross references: plus the room to add your own that are missing. None of the cross-reference resources are very complete for deep analysis of hearing God, walking in the spirit and the like.
• Good maps: though you need a good Bible atlas too (though i haven’t found a good one yet).
• Word definitions either in the footnotes or built into the cross references: I know that I often am forced to use STrong’s, Vines’, and greek and hebrew lexicons, but having them already there on the page is very nice.
• Wide margins for taking notes: this is essential. Nothing beats the Inductive Study Bible for this but you’ll be forced to use the NASB—however, that is a very excellent literal translation, even though it’s not real good for reading.
• A translation you trust: You’ll need to pick up many translations and paraphrases, but unless you are certain you will use them for serious study, they can be paperbacks or hard-bound covers
• No denominational slant in the footnotes and commentary: you want clear explanations of the scriptures. You do not want doctrinal slants.

What else do you look for?