About the only thing in common between the two would be the engines. (If we are talking about current models of course.)
The 109 as a corporate aircraft would probably be the better of the two.
The issues and which is better (IMHO):
Speed - 109
Comfort pilot - 135
Comfort pax - 109
Visibility - 135
To fly - 135
Maintenance - 135 by a long way
Range - 109
Payload/Range - 135
Single engine perf - 135
Noise - 135
Avionics - 135 (MEGHAS version)
Reliability - 135
Water leaks - 135
Build quality - 135
Resale value - 135
Number in service - 135
Contemporary - 135
Pax safety (clearance from the turning bits) - 135
Crashworthiness - 135
Based on currently operating both aircraft and in our particular case. A109E and EC135P2. The 109 has 1000 HP for T/O versus the 135's 842 HP.
The increased gross weight mod on the 109 is probably 250K USD to retro. The new T/R blades about 25K USD to retro depending on the residual life remaining on your current blades or could be as much as 50K USD if you are replacing expired blades.