Marketing mailshot or telesales

[Will]

Hello!   Ah, the age old question...
 
I'd suggest an integrated approach as Brian said.  e.g.
 
* Personalised direct mail letter (if it's targeted properly and well-written, you'll get sales - otherwise why would companies spend billiions every year on direct mail letters?)
 
* plus a leaflet or brochure can also be good as an informational aid "A letter sells, a brochure tells")  
 
* plus a scripted followup by phone as soon as practical after sending the letter...say 5 days?   The longer you leave the followup, the lower the response.  
 
Other things to consider...
 
1.  Test different letters and approaches until you find something that works.
2.  For complex services (e.g. B2B) consider building a longer sales cycle (e.g. first letter invites prospects to send away for a more detailed info pack.)
3.  Everything comes down to execution.   A rubbish direct mail will get a rubbish response.  A great letter with a great offer will get a good response.  
4.  The letter then call format is good for warming up prospects a bit.   Not many people like cold calling from scratch, but some seem to do well.

[Business Brian]

"A letter sells, a brochure tells"
 

 That's another one for the quote box.