Depends on what your strengths are. Are you willing to do more body work than engine work?
I have found the I4 2.5L to be very reliable. My 2.5 cranks out 140HP (87 Octane, E-10), 146HP (98 Octane, E-85) or 154HP with (E-100, 112 Octane) with a bore and Weber 34DGEC upgrade kit. The lead shop tech who did the work on the block said it was the best 150 he had ever worked with despite the damage from the previous owner. Its bored to 2.7L.
As for the I6 4.2L...
Its a good motor, more modern and lots more mods. So if you are more comfortable with body work get the I6. The I4 is just to hard to eek the same HP out of it.