With commercial ebooks, all formats are typically produced from one "master" format. So the question comes down to what is lost in the conversion. Both LIT and MOBI are based on the OEB "standard", but LIT does less to the OEB original than MOBI does. So I prefer LIT when I can get it. This is particularly the case if you may want to later convert to an even richer format like ePub.
There isn't typically much difference these days between how new MOBI and new LIT ebook will look on the screen. However MOBI isn't a single format - it has evolved over time, and older MOBI ebooks have limitations (almost no metadata, very small images). The problem is that you can't always tell until after you buy an MOBI ebook whether it is in the new or old format. So if you have a Windows PC, LIT is still the safest choice.