I may be 100% wrong as I haven't flown 135, but I think the instructing would be safer. In a well maintained 152 and 172, for instance, instructing is very safe. I could picture a VFR charter operation really pushing the limits on weather, but I guess it all depends on their mission and where in the country you would be flying.

Doesn't do any good to build hours fast if it gets you violated or killed.

Good Luck with your choice.