Differences Between Shimano Ultegra & 105 Drivetrains I'm in the market for a new bike (or possibly a groupo upgrade on my current bike that has 8-speed Ultegra 600 components from the early 90s). I'm trying to determine if the premium for an Ultegra drivetrain is worth it. 

I know one of the main differences between Ultegra and 105 is going to be the weight, which I am not as concerned with as much as I am with durability and reliability. 

I typically put 150-200 miles a week on my bike with my long rides being 50-100 miles. I am not a serious racer, but I do participate in 3-4 organized race/rides each summer. 

If I go with a new bike I'd like to stay in $2k range, but could talk myself into as much as $3k if warranted. 

Also, since many builds seem to be coming with mixed components are there any items I want to make sure I don't skimp on? I'm guessing STI levers and derailers specifically?

Any words of wisdom?
If you're planning on doing a reasonable amount of Kms I wouldn't purchase anything with less than 105 on it. The groupset tends to reflect the quality of the bike your purchasing. If it comes with a mix of 105 and Tiagra, then it is probably aimed at the recreational rider who might take it out once every other week.

As for 105 vs Ultegra, some parts may make a difference. I don' know the full implications, but I would really take a 105 and ultegra out for a ride and test the shifting speed and precision. I expect the ultegra will shift a little quicker and smoother.

Personally I run SRAM force and love it. It isn't for everyone though as there is only one paddle that shifts up and down based on the length  of your push. With that said, on the cassette I can shift up 3 gears at a time; taking me from 12 to 32T in 4 throws of the lever.

I know in Australia $2k will get you a reasonable bike running 105; and we are one of the most expensive places in the world to buy bikes.