The Tang Band W5-1138SM 5-1/4" Neodymium Subwoofer is an amazing product for it's size and cost, making it an even better value.
Your question of a sealed or ported enclosure is truly subjective. If you are building your cabinet and have good woodworking skills, I prefer a ported and tuned enclosure, but if not, then you will receive good sound from a non-ported or sealed box of proper size.
A sealed cabinet will be strong and provide a tighter low end with solid transient responce while a ported enclosure will go a little deeper and be punchier with less applied power than a sealed box. I have a DIY system that is actually 2- sub woofers in 1 box with 2 chambers. One chamber is a similar speaker to the Tang Band W5-1138SM and it is sealed, the other is ported and contains an Eminance 15" down firing with a front tuned port. I added a good sealed back mid and ribbon tweeter for clean mids and highs. This speaker has punchy deep slammin bass, smooth mids and transparent highs. The point is that I designed around my taste and I love it. HOWEVER, an audiophile friend feels it is not as accurate as it should be. Well accuracy for him bores me. I like everything from metal to jazz fusion and even big band music recorded well. I like it exciting.
Build to what you want to hear and you will always enjoy your DIY, don;t consider other's ears because you are the expert.
ENJOY! Good Luck