Well - for my two cents worth...
I had the same choice and, after much debating on what the most sensible thing to do was, it all came down to what I wanted out of life at the time and for the future.
I was very unhappy at my current company - working very hard multi sector lo-cost shorthaul and no life at home or outside work in general. Command was offered, assessments passed and start date for the course given. Meanwhile, alot of my friends in the company left for longhaul pastures new. No worries, I thought - take the command and at least the extra money will compensate for the rest of the pain......but for how long? And once those financial handcuffs are on - how easy will it be to take what will equate to a big pay cut to leave LHS and prob go back to RHS with a new company? I had also managed to secure a job offer from a longhaul carrier - just to make the decision harder! Many more questions filled my head every night for a considerable amount of time and all seemed to make the choice harder.....time to sort the wheat from the chaff!
So...it came down to this...WHAT was making me unhappy at my company? Answer - lifestyle due working hours. Would this change with seat change? Answer - no! If I take the command, will I be happy putting up with this - not in just the first couple of years but in the next 5, 10 or 15? Answer - no! Will the command help me in a seniority based long haul airline? Answer - not really...still start on the bottom of the seniority list with 2 stripes! Will I like longhaul? Answer - don't know!
Now, stay with me... the crunch bit....can I go back to shorthaul if I jump and dont like it? Answer - Yes! Will a command still be available. Answer - yes, either straight away or within a year probably. If I stay, move seats and am still unhappy, can I still jump ship? Answer - no, job offer will be gone! (and we all know how often they come along in many companies!)
So....I jumped ship!
Was it the right move? Well, I am sat writing this in LA after which I have 10 days off at home(where home, by the way, is now a far more sunny part of Europe than when I was at my old job!). Sure, the company has its problems, but so does ANY airline. Command is years away at present but suddenly, on the other side, that does not seem to bother me now I have what I wanted out of my life back - I have even started integrating back into the outside community and begun forming tentative friendships!!!! Will I ever go back? Not right now but who knows....at least I have the options.
So, as I said right at the top of this post, I think it all comes down to what you want out of life and your job personally. Sure, there is much advice around - you can build that CV and get command time on there, never turn down a type rating etc etc etc.....
Listen to it all and then think this....What will make YOU happy? Remember, great as this job is...it is just a job....only a part of your life, albeit a big one!
Don't know if this helps...it is just my perspective on what was, at the time, a difficult choice.
All the best, whatever you decide!