The 200mb for 30 days plan is available by default in for all tablets if you sign up for t-mobile no matter what you do and you basically can't get rid of it. It can be overridden by other plans but when they run out the 200mb plan goes back in effect. The 5GB for 5 months is a pre-paid plan and is not available to post-paid customers. I found that out when I had to switch to pre-paid to get it.
I'm only speaking for myself here but here's what I see happening. I got an iPad from T-mo a few months ago and signed up for the 2Gb plan on the spot. Each month the iPad automatically uses the 200Mb first and then switches seamlessly to the 2Gb. I can see this happening every month under Settings - Cellular Data -> Manage T-Mobile Plan.
I don't know if this is normal. I don't know if this will happen to you but that's what happens on my account.
You can get the free 200mb plan right from the start. I did that myself, but since I will be traveling and using my iPad and already ran out of the 200mb. For a one time charge of $10 and 5GB that you have 5 months to use, you really can't go wrong with that promotion while they are offering it.
The 200mb plan is for life - you get 200mb every month. When you sign up, you automatically get the 200mb plan. With the 5GB, you first use up the 200mb for that month, then it starts to draw down the 5GB.
I have the 200MB plan only right now and have had it active for about 2 months. I can still see the 5-month promo plan on my list of options (and will probably pick it up eventually). The way their services work, if you buy some sort of data plan, the 200MB/month will be tacked on to that and reset every month and when you stop paying for any extra data, it will go back to 200MB/month. Pretty nice deal, as it keeps the SIM card active and gives you a bit of data to fall back on (Verizon SIMs supposedly expire after 5 months of non-use, and AT&T/Sprint expire after a year of non-use).