If You promote by winning the championship You get a promotion bonus where basically You can set Your prices higher than usual and Your arena will fill up until the allstar break. Also the tv contract is a lot higher here. So if You stay at the salary floor and remain in the cup You can have a roster that will win most div III leagues and maximize profit. So even if You relegate, You relegate with a high draft pick that will usually turn into a good prospect that You can sell or train and with loads of money. I was actually staying competitive in div II earlier this season with a 250k roster. I was making 180k a week plus 50k from cup.
To summarize: If You have a chance to promote, take it, its always the better option.