My suggestion is to save money now and get the tank that will be better for the long run. I have never kept sharks, but from what I can tell, the people with the most success set things up correctly at the start. They are messy eaters, but require excellent water quality, so a bigger tank with very good filtration will be necessary.
Why the hurry? In my mind, you are spending the money to set up 2 tanks, when you could be focusing on only one. Further, if you start acquiring bits and pieces and keep an eye on the classifieds, a little patience will save a ton of money. Plus, if the smaller tank doesn't work, you have to buy a second shark to replace the one that dies. I have never regretted going slowly, and have often regretted haste.