i keep my power meter on my race bike all the time for two reasons:

1) i can review my data later
2) when ur going crazy in the first 20 minutes of a race trying to get into the winning break (most local races'winning break are set in the first half), i can quickly look down at my average power and see that if its over 350 watts for the first 20 minutes maybe i should chill out

My input though would be get either a powertap or a quarq, or wait and get an srm (srms are major bling, but i could never find any cheap enough). A power meter will completely change your training outlook, and hopefully will cause you to train smarter, and get stronger. di2 on the other hand will just make ur perfectly shifting bike shift a tiny bit better.