Joco,

it seems you have gotten yourself in a fix.  Personally I think that the best route is always the honest one.  I think you would be better off going with agency #2 and being honest and up front about everything.  I think it shows badly on agency #1 that its own officers told you to lie about something as miniscule as one use of Marijuana.

When I first began my trek into LE I received similarly bad information, but mine was more along the lines of admitting to some " youthful experimentation" with drugs, because it looks very suspect to deny having used drugs.  Unfortunately, I followed their advice.  I  didn't get hired for the department and later found out it was because they had disqualified me for drug use within the last 5 years.  Yes, I had managed to disqualify myself for a job I really wanted by following the advice of other cops on that department and stating I experimented with a controlled substance that I had not used.  Had I been honest from the beginning then I would have more than likely been hired.  

We live and learn, and thereafter I had to explain to other departments that I had originally LIED to the first department about my drug use and that in fact I had not used drugs.  (I can imagine how this made me look) Well, I have a great job now and have not looked back until today (briefly to hopefully help you).  While it eventually worked out for the best, I highly suggest you start your career HONEST and don't follow anymore advice.  The honest route is the best one.  This isn't advice, but a fact when applying for LE jobs.

Good luck!