What Bike Is Next?

I'd say "welcome to the sport" but...10 years makes you quite experienced. 

A couple things to consider: 

Bike fitting-Yes, a bike fitting (and maybe 2) is likely to be a good investment.  Your fitting might change a bit as you ride more (hence 2 fittings) and get more adapted to the riding position.  Expect that you may find muscles you weren't aware of in the weeks after a fitting. 

I saw mountain bikes at the HIM's.  Hardly efficient, but...if that's what you've got, RIDE IT!  Humbling anecdote.  I had just gotten my tri-bike and met up with a friend who was riding my old road bike.  For whatever reason, I had to go back to the car within the first mile or so.  I was riding "hard" on my tri bike to catch back up to my buddy and got passed like I was standing still by a guy on a mountain bike!  (I was probably pushing 18 mph, so the mountain beast was going 20-22.) 

The guy at the bike shop makes $$ selling bikes!  Of course he (as well as the glossy triathlon magazines) would like for you to drop coin on fancy gear. 

I've got both road and tri-bikes (and am commuting these days on my old mountain bike).  Since 2006, I've put about 5500 miles on my tri-bike and about 6500 on my roadie.  I've done 3 Iron-distance races.  I do the bulk of my riding on my road bike.  I had my tri-bike set up on the trainer when I needed to ride indoors, so "off-season" I'd do 45-120 minute rides on it.  I'd usually try to get 3 centuries in prior to an IM.  I might do mostly road bike rides up through the first century, then the shorter rides on the tri-bike and second century on the roadie and the last couple weeks all on the tri-bike. Anecdotally, switching to the tri bike was good for 0.5-1 mph on 18 mph rides. 

Think a little about what tri-courses/races/raining routes you might do and what your personal riding style is like.  If you're riding pancake flat routes and are disciplined to sit on the aerobars, you'll benefit from a tri-bike (or at least aerobars added to your roadie).  If you're riding rolling hills (or like me, can't camp out on the aerobars for an hour), you won't get as much benefit from the aero position (hills break up the wind more, and you'll give up power to try climbing much in an aero position). 

Upgrades-I've got clip-on aerobars on my road bike.  Those would be the biggest bang to extend the life of your current ride.  If you're disciplined, an aero-helmet would be next on the list.  After that, aero wheels that you could swap to a tri-bike. 

Of course, if spending $2500-4000 (or more) on a spiffy tri-bike is acceptable to your "sponsor" (I.e. you won't miss your mortgage/rent/car payment spending that money on a bike), fire away!