Hi Steve, me again!!!
I see many people responded and gave you a lot of good advice. I was thinking about your dilemma about the trainer last night (yeah, I had nothing better to do at 4 am when I could not sleep) and decided to share my experience with Loki's trainers (note the plural) over the past 3 years. After you read this you will most likely conclude that I am "insane" and kind of stupid ... I already think that about myself

Loki's first trainer used all positive reinforcement techniques for treating dog aggression.Thank goodness for that because no harm was done and while Loki did not progress he did not regress either. After a few months she went on a mat-leave. So no progress and Loki still reacted aggressively towards new dogs.
First lesson learned from working with the first trainer: Following the text book examples without real life experiences is not very productive!!! This lesson was about CDN$1,000,00 over the 4 month period.
Status-quo was not acceptable to us so we we decided to call in the second trainer. We worked with her when we got our puppy mastiff-mix. We remembered she used positive reinforcement techniques with our puppy and she was well known and respected and she was known for being successful with dog-to-dog aggression. She suggested we board Loki for 4 weeks with her. We trusted her and we followed her advice. In those 4 weeks there was not a day I did not miss Loki and I could not stop thinking we made a mistake by giving him to her instead of working with her and Loki in our house. 4 weeks later Loki was back - with a prong collar, a perfect heel command and the advice to "keep him away from other dogs because our dog Darko should be more than enough for Loki to play with!!!!" So, the heel command lasted only as long as the prong collar was on Loki and guess what. I could not use the prong collar. I walked Loki on it for one day and the first time I had to correct him the way the trainer showed me and I heard him yelp it broke my heart. I took the collar off and threw it away. For those of you reading this, please do not take the comment about the prong collar the wrong way if you are using it on your dog. If correctly used this collar can be quited effective. I just could not use it because I tried the "tug" correction on my own skin and I did not like it. So we all follow our own believes and gut-feelings.
2nd lesson learned: Do not give your dog away and do not follow advice with which you are not comfortable. The price tag CDN 2,000. Loki was still lunging towards unknown dogs.
The third trainer (now we are almost 1 year into dealing with the dog aggression) is the same trainer that Carol mentioned; Loki and Trouble go to his class together this month. By now you know that Jon uses the positive reinforcement techniques but with an instructive reprimand (change in the tone of the voice). He worked with us for about a year and he managed to get Loki to accept meeting new dogs but Loki was not consistent in his progress. Huskies and German Shepherds remained the biggest challenge. We have also noticed that Loki responded to him much better than to us - even though we would go for the training walks together. When Jon was handling Loki on those walk Loki was "the perfect dog". When I was out with him - oh well, you can just imagine. So the problem was with me and my husband and all the wrong signals we were sending to Loki i.e. our body would tense and I would pull the leash etc.. Eventually we stopped waisting Jon's time and I started to read the books about dog psychology and training; I am still reading like mad.
The third lesson learned: I had to learn how to work with my dog.
After about 10 months of just simply "managing" Loki and keeping him away from other dogs I was ready to try again.
The fourth trainer started to work with us in Dec. 2009. She developed a very structured and well thought out approach to Loki's issues. It was her idea to get Loki ready to start attending classes with other dogs. But before he could go into a class Loki had to learn how to manage his anxiety which was apparent but we all missed it!!!! So the root cause of Loki's fear was his anxiety about anything new, including dogs.
Now we are attending her classes and Jon's classes. These two trainers have different styles (not techniques) and their styles combined are finally producing very positive results. The key in producing these positive results is that while the trainers are guiding me and showing me how to train, I am the one working with Loki. Loki and I are learning together and that is also allowing us to trust each other.
I was the most comfortable following Jon's style but I also learned that the fourth trainer's style was actually giving me more structured approach which I needed (more than Loki :-).
So this is a very long story (sorry about that everyone) but hope it illustrates how important it is that we learn from the trainers and we work with our dogs. On top of that, in my experience it was absolutely critical that I was comfortable with what I was doing. One more thing that Jon thought me and it was very important for me and Loki: "do not be afraid to try different things until you find something that works for your dog but always be kind to your dog". So do not spend thousands of dollars like us - find a good trainer and work with Safi; it will pay off in many ways.
Betsa