Originally Posted by Jeff
I can give you two answers, that hopefully will be sort of helpful.
1) Ride them. If it's the same size frame and they're set up the same, take them out for a spin and see if you notice the difference. There's other factors to consider - "better quality" stuff tends to hold up better, too, within limits (ultra-light componentry may be an exception, but you know what you're getting into at that point) - but for casual riding, if you don't notice the difference on a quick spin, it's probably not critical.
2) Generally, two things happen as you spend more on components. Really
cheap stuff tends to have a lot more plastic and to be made to somewhat lower tolerances, so initially as the price comes up, you're getting more durability. I suspect you're already over this threshold, though. Past that, generally what you're getting is weight reduction. Going from a Shimano 105 rear derailleur up to Ultegra, aside from the obvious cosmetic differences, will drop you from 250g down to 195g, a savings of about 1/8lb, for going from a ~$25-35 to ~$60-80 part. You don't want to look into the price hike up to Dura-Ace.
So, as stuff gets more expensive, the overall functionality is pretty consistent (though, you do notice improvements there, too - I went from a Tiagra/105 hybrid to full Ultegra, and shifting definitely got a lot tighter and faster on the better gear, and the Ultegra brakes were so strong I was afraid I was going to go over my bars the first time I hit them, lol) you're going to start seeing some pretty big weight savings.
Whether or not this is worth it to you depends on your goals - my dad gives me shit every time I buy lighter bike stuff, because "now you're going to have to work even harder to get the same amount of exercise," so if your concern is fitness and not speed, and you're not planning on really thrashing your gear, then it might not be worth it. On the other hand, there's no doubt my ~17.5lb bike with stiffer wheels and cranks makes me a bit faster in the hills than my old ~25lb bike, so if your objective is to try to pick off the KOM on a local hill climb, it can definitely help you on the margins.
You may get a range of different advice here, but I think personally I'd take a really nice frame where they cut corners on the componentry a little, than a lesser frame with nicer components. You can always selectively upgrade that stuff down the road, but you're stuck with the frame.