I have just bought a new OCR3. 8 speed with triple chain ring. The shifters are shimano sora flightdeck. The derailleurs are sora.

My old bike was an Avanti Kona, with shimano 105 running gear. It was a nice bike, but a bit old and heavy.

I am wondering if the derailleurs on the old bike (105's) would be better quality than the sora ones on my new bike, and if so would they be compatible with the sora flightdeck shifters.

Could i do it myself and would it be worth the hassle.

Advice welcome.

Thanks

the 105 drivetrain is better if you compare the same years' products. A bit lighter, closer tolerances but other than that, almost undistinguishable from each other to the casual user.
If there is more than a few years difference in date of Manufacture, the difference is insignificant or maybe even in the favour of SORA.

Is the 105 group 8 speed or 9?
I'd say switch it over if it's 9 spd.

Was it 105 from 10 years ago (105SC, which was 8-speed at the time) or from 18 years ago (6-speed at the time), etc.? Was it pre-indexed-shifting 105?

If your old bike has indexed-shifting 105, then you could swap out the deraillers. All Shimano indexed rear deraillers (except pre-9-speed Dura-Ace) have the same cable pull and are thus compatible.
Front derailler is more tricky. STI shifting is indexed front, and requires special cable pull for the front derailler, that 105 has had since 2005. If your old bike has STI shifting, then you'll probably be fine.

However, I don't think it's worth swapping out your new Sora for your old 105, unless the 105 is relatively recent. The Sora will work just as well, if not better.

If the 105 rear derailer is SIS, it will work with the Sora shifters. If the front is STI compatible, it will work. But 7-speed 105 is probably NOT STI compatible. I could not get an old 105 FD to work with Ultegra STI shifters.