Variant Study

Are you looking to work with the greek texts directly or are you looking for a summary of the differences?

James R. White's King James Only Controversy does a great job of summarizing the history of the KJV and the text behind it, and then summarizing the textual differences between the KJV and modern bibles like the NASB or NIV. He has an entire section titled "Textual Data" with actual side-by-side examples, and he has a good scripture index in the back of the book. D. A. Carson's King James Only Debate is another book that summarizes the different textual traditions (text types), summarizes the history of the KJV and the Textus Receptus, and gives some info about how textual decisions are made. This book is a bit older than White's book but still fairly accurate over all. People can benefit from the info in these two books even if they're just users of the KJV but not KJVO.

Metzger's Textual Commentary gives an explanation of all of the major textual variants and the evidence for the chosen reading of the critical text behind most modern bibles (the Nestle-Aland critical text). Omanson's Textual Guide is a less technical version Metzger's commentary designed for people "who have not received formal training in textual criticism."

Of course if you want to work with the greek text itself you'll need the NA28 with the Critical Apparatus, the TR (of which there are several versions available in Logos), and the Byzantine text. I'd recommend you get either the Logos 8 Academic Standard or Academic Premium base packages since they have all the necessary greek texts and both of the textual commentaries mentioned above. Premium also has Philip Comfort's Textual Commentary, the Text of the Earliest Greek Manuscripts, and the Anchor Yale Bible Dictionary (which has a great article on NT Textual Criticism that you should probably read). Depending on your dynamic pricing you might save a ton of money doing it that way. I'd also highly recommend you get the User's Guide to the NA28, it's only $9.99. You'll want to read that along with the "Introduction" within the NA28 itself so that you can know how to use the NA28 Apparatus.

Hopefully I didn't provide too much info but I wasn't sure how in-depth you wanted to go and whether you wanted to get into the greek text itself.