MKT,
I have written the formula to compute the tilt based on the cell range you need to achieve. Check the related post.
http://www.erlang.com/forum/erlang/thread.htx?thread=4196
Standard practise in urban environment : 6 to 8 degrees tilt.
Electrical vs. mechanical : easier to set up, easier to modify, backlobe is tilted as much as the front lobe, less visual impact, remote adjustment possible.
So it is good to buy antennas with default 8 degrees electrical downtilt.
If mech and elect are put together (this is oftenly done), you just sum both tilts. 4+2 = 6 degrees of effective downtilt.
So if 8 degrees elec tilt is too much, you can always apply some uptilt. This will further reduce the backlode 🙂
Regards,Pix