In my understanding, the difference between the two ways you described above mainly exists in the following:
The first way is to use the second TL-MR3420 as a wireless access point or wireless switch which is wiredly connected to the front-end network; while the second way is to enable the WDS function on it which will be wirelessly connected to the network. The WDS function makes the second TL-MR3420 picks up the wireless from the first router and at the same time broadcast its own wireless network. Here is an instruction on WDS settings on TPLINK router: http://www.tp-link.com/en/article/?faqid=227.
In fact, I much prefer to the first way to setup the wireless network with the same SSID and password. In my opinion, the hardware cable connection between the two routers make the whole network connection more consistent.
Besides, you mentioned "seamless roaming from one router to another". Whether the seamless roaming can be realised or not is determinded by the wireless adapter of your wifi devices. If the wireless adapter supports seamless roaming feature, then a seamless roaming in the network is possible.