Generally, unless you are using heavy XML (InDesign does XML but not as advanced) or DITA (which ID does not support), it is a heated race. If you are doing advanced XML or DITA, then Frame is the way to go.
Frame's conditions are slightly more advanced in that they can be applied to table rows; ID's conditions can only be applied to the entire table. Other condition features are similar. ID's Indexing is slightly easier with use of it's topic list in the Index panel, but I believe that ID only allows one index per book. ID supports advanced OpenType formatting (and OT fonts are cross-platform, so you could use ID on Windows). Other notes... As mentioned, ID is more sophisticated in the typography and layout departments. ID has GREP find/change, GREP supported formatting, nested styles. ID cannot span columns with subheads if working with a multi-column layouts, and does not have run-in heads. InDesign's table styles are more complicated, but more advanced above Frame's. ID is more advanced when importing Excel files, plus ID can link to Excel files for auto-updates. ID's table styles do not contain any geometry (size) information, but there is a plug-in available for that, and to not generate automatic table titles as part of the table style. Frame's variables are slightly more advanced, especially when concerning using character styles. (ID does not have the table continuation variable) InDesign has a separate product called InCopy for allowing others to edit text while the InDesign file is being worked on. (InDesign can handle all the text edits too.) InCopy can be used to generate new text, as can Word. InDesign's PDF export functions are more advanced. (Current versions of CS3 and CS4 do not have a size issue with PDFs.) This is not a complete comparison by any means, but should be enough (with the other posts) to form an opinion. There is a converter for MIF files to ID at: <http://www.dtptools.com/product.asp?id=mfid> Although I have not updated it to Frame 9 and IDCS4 (my bad), I have a comparison chart at: <http://www.ideastraining.com/PDFs/SelectingDTPprogram.pdf> As a Mac user (and certified Apple Consultant), I would take a hard look at using ID on Windows with OT fonts to keep the client happy. That being said, I use Frame with Boot Camp and Parallels all the time. As long as the client has a reasonably fast computer with 4-8 GB RAM, they should be fine. If I didn't address any specific issue, please let me know. David Creamer IDEAS Training & Consultation http://www.IDEAStraining.com Adobe Certified Trainer for Photoshop, Illustrator, Acrobat, InDesign, InCopy, FrameMaker, Dreamweaver, Premiere, GoLive, and PageMaker Authorized Quark Training Provider Enfocus PitStop, Markzware FlightCheck, & FileMaker Authorized Trainer