Which would be a better buy and why-Colormunki Smile or Spyder4.0 Express?
The Colormunki Display or Spyder 4 Pro are both quite a bit more flexible, and cost only a little more than the Colormunki Smile.
Neither can profile printers. Do you print with your own printer? If not (e.g. if you get prints done by a print lab), no problem, these will do fine for a monitor. If you print with your own printer, and you use the printer maker's ink and paper, then the printer maker's profiles (usually installed automatically when you install the printer driver) may be good enough. In my experience with several Epson printers that's been the case.
If you're really serious about closest match of colours on your own printer, you need to go up a grade to something that profiles printers - e.g. Spyder Print or Colormunki Photo (at double the price). But profiling the monitor is (IMHO) more important.
By the way, for either of these, the software provided is not as good or flexible as Argyll (open source) software. Argyll software is a bit of a pain to install, as you need to install the basic software from http://www.argyllcms.com/
but this is command-line only. There is a very good Windows, Mac or Linux front end called dispcalGUI, which you get separately from http://dispcalgui.hoech.net/
. However, I would strongly recommend the Argyll / dispcalGUI combination over the provided software for pretty much any monitor calibrator/profiler.
PS - just seen Czornyj has said much the same thing, only much more succinctly!