It's not really a core that matters in most cases but peripherals. Most embedded veterans can start being productive on new micro in a day, they all do pretty much the same things, ADD, SUB, jump, branch etc.
But what really matters is if you have A/D, D/A, SPI, I2C, USART, PWM, timers/counters, CAN, USB, Ethernet etc. Microchip blows others away, because instead of giving you a micro for everything, you can choose a micro that has only peripherals that you need for half price. And you can drive LED directly, most micros need a transistor for that.
Of course, sometimes the speed is important and that is the core function. But if one gets to squezing last nanosecond out of the micro, they should have started looking at faster models long time ago.
8051, PIC, Freescale ... is irrelevant. Toolchain availability and quality and peripheral availability is what matters.
PIC has many peripherals available, many sub-level quality but price friendly toolchains and inexpensive.
8051 has good quality toolchains and many vendors who licensed the core from Intel.
Both of them are good.