Well lets see,
Costs: The 3.4L swap will most likely cost less. With the 7MGTE there will be lots of cutting and fabricating. I've seen a few of them but i know for sure that you will need hood pins because the 7M is longer and you will have to cut your upper radiator support. Also i have not seen one were the A/C was installed.
Dependability: Well i'd say that these motors rank about the same. (for a daily car, not racing etc...)
Power: The 7MGTE block will of course handle more power. The 3.4L stock is 190HP. I don't know what the hp is for the 7MGTE but i'd guess around 200HP.
Overall If you just want something more powerful to pass cars, pull trailers, or simply go fast, and have a factory look with a lower budget go with the 3.4L. If you want more power to race or whatever then the 7MGTE will be ideal since you can modify those motors up to 1000HP with very little internal modifications.
Mike