There are lots of factors in choosing the right rosin....
Different climate (in less temperate climates to the UK people may have summer and winter rosin, as it tends to get more sticky in hot weather)
Amount of dust it makes....some people are allergic to rosin dust! And it makes a mess on your fiddle and strings.
Good match with your strings....this may be a bit trial and error, but I now use Obligato rosin as I have Obligato strings. I suppose it should make a difference, maybe if I could play better I would be able to tell
Some have gold or silver dust in them too....not sure what this is supposed to do (unless it makes your fiddle look like its been coated in fairy dust after you've played)
Rosin is usually very cheap and lasts like forever. Usually you end up throwing it out as after a while it can become dried out and brittle and not so effective, rather than actually use up a whole cake
Tartini though is really expensive and well recommended. It is supposed to give a good grip etc. Apparently the manufacturers have stopped making it now for good, and there are only a few blocks left on the shelves. Buy now while stocks last