conclusion for 8051
AVR is a good architecture from one supplier. 8051 is much older but supported by many suppliers. The fastest 8051 devices from Silicon Labs are MUCH faster than the fastest AVR8, the slowest 8051 devices are MUCH slower than the slowest AVR8. In other words, performance with 8051 covers a huge span while AVR is much narrower.
In regards to noise immunity and other conditions you listed, that depends much more on a silicon process and packages than it does on an architecture. There are excellent 8051s in all those regards and there are not so good ones.
My recommendation, if you really have so much experience with 8051, you can find much more variety of devices for 8051 than for AVR. If you are starting something new and are willing to compromise here and there, I would think that the AVR is the more modern (better) architecture. If that warrants switching from 8051 to AVR, that is totally up to you.
Bob