Speaking by experience here,
I've done lot of conversions from ePub to Mobi and Mobi to ePub with zero problems all the time. The common factor is that most of them have been pure text or novels with just a couple of pictures, but still zero problems after conversion. I've even converted two SQL books (they usually have some graphics and complex formats) and everything looks fine.
My suggestion would be, check the biggest online store in your area or country and compare to Amazon. If you can find more of your favorites titles in Amazon, go for it, and get a Kindle 3. For a pure text book the differences in display between ePub and Mobi are zero! They look exactly the same, native and converted, that's my personal experience.