i have many 1000's hours on both types of aircraft. I prefer the boeing. It's a proper aircraft that flies like a proper aircraft all of the time and not just when 'the computer says no' and hands you an out of shape computer.
But honestly, once you have command hours they can never be taken back out of your log book. You are marketable worldwide on the 2 most popular types of aircraft currently in operation. Plus if they are offering you a LHS now, who knows what might happen to the companies economic standpoint in 6 months time.
After 6 years in the RHS, if you don't think you are capable of running a structured day in accordance with SOP and company procedures and know what it's like to do the job, then I would seriously consider your career progression to this point. You should be a more than compitant SFO and should be comparing your mental model to the LHS pilot on all aspects of the operation. Take each standard day as if you were the captain and ask the LHS if you can take the lead to see if your thought processes are the same as his. This is the only way you can start to reduce the workload associated with chancing seat. You should know how to fly a sector, manage a crew, and manage a turn around before even getting near LHS. Then all you have to get used to is the change in type and using your right hand instead of left for playing with the MCP/FCU.
I can completely understand your trepidation about changing type and seat at the same time, but believe me that changing operator SOP and type (regardless of DEC or DEFO) is a lot harder then type and seat within the same operator.
Good luck, but my advise is as above, ALWAYS take a command when offered!