Look at sibling comment and decide if you really need an interactive whiteboard or if simply using a normal dry wipe and taking photos and then tagging those in a photo gallery program systematically somewhere will be good enough storage/retrieval. My students still take photos of key screens even though I email out a pdf of the IW screens after the lesson.

If you decide you really do need the IW, then try to find a local school/college that will let you play with theirs for an hour after classes. You will get a good idea of what the software can do, and you can find out about the drawbacks of various makes.

One concrete advantage of an IW might be annotating actual screens. Most IW software automatically grabs screen of driving computer then allows you to write over the static image. All the IW software I have used (smartboard, active edition, the dolphin ones) allow saving of whiteboard stacks as PDFs.