. . . and have pretty much settled on the Expobar.
Why? Considering your price range, you should take a good look at the Bezzera line here -
http://www.1st-line.com/ . And, of course, there's always the grinder - this is a nice one - Baratza Vario - available from a variety sources.
But, to answer your question, the flow controls add a little bit of complexity to the machine's innards and to the electrical control scheme. Personally, I would prefer the single button and let my senses determine when to stop the shot.