A better question would be what style are YOU suited to. Some people are comfortable on the ground, others aren't and like stand up. Rolling creeps some guys out because of some of the positions. Ultimately picking a style you're suited to will enable you to focus on and get better at that style.
With regard to your question, "What are the pros and cons of muay thai and BJJ in a 1v1 fight?" IMO this answered by the very early UFC fights. Back in the day most of these guys had one discipline with very little cross-training. The Gracies and BJJ dominated.
If your looking to train with your friend, why not join an MMA Gym and train a variety of things?