The difference couldn’t be larger, it’s a different universe.
Mostly domestic vs mostly International.
Flying 135 I’ve been to every state in the lower 48. Never realized how impoverished certain regions in the US are.
Check the regs, 135 PIC time does NOT transfer over to 121, unless you’re flying scheduled 135.
Depending on his/her age your friend is at a crossroads, 121 or 135 for a career?
Family’s plans?
My career progression was 135 > 121(s) > ACMI and I couldn’t be happier.
Never regretted my choice for even a single day