There are cases to be made for both,
which depends on many factors including your internal marketing
desires, CRM software, POS capabilities, etc. I will give some key
points on the one code topic for now, but for more details on what would be best for you and your campaigns please speak with your account director.
One code for everyone works well, and is almost
exclusively used in all campaigns. It's simpler for back end
processing as you don't have to shut down each coupon as it is redeemed
and POS doesn't have to be live, can significantly aid in viral
campaigns, and if POS or CRM software allows you to instantly access
your text message database you can employ the forced opt-ins for
redemption. Also, you're not giving out such a good deal that if
redemption is high it will put you out of business, and high redemption
will attract / entice text responsive customers to opt-in. Rewarding
customers who forwarded this text message is also possible, as the
number the message was forwarded from now changes from your short-code
to the sender's, and by asking for the senders number (who is probably
already opt-ed in) you can now track how many redemption's were
generated from that specific customer and reward them with a special
text coupon, gift certificate, etc.
For Catalog and Internet only companies, all of the above and more,
tied with good message timing and a short time frame (which isn't a
problem with text, 80% read rate within the first hour) keeps Internet
posting of coupons to a minimum and in all actuality, is not a problem
anyway because of the option of forced opt-in's for redemption!