I used to have both. The $30 T-mobile plan was a dream come true. I had reliable speeds, unlimited texting, but both myself and my girlfriend hated the latency issues I had on wifi calling. If you're not the biggest talker in the world then I highly recommend this plan. But, if you do like to make lots of calls on your phone then I would try every wifi /data calling set up possible to find one that works for you; I was just too impatient with it to get it to work well. There's nothing worse than cutting someone off because you didn't realize they had started talking 4 seconds ago due to horrible latency issues.
The Straight Talk (At&t) plan was working okay, but their APN settings they gave me to enter were pure garbage. I couldn't get data to work for a couple of days until I Google searched around for better ones. Once I got that squared away it was overall a good service to use. I just didn't like their customer service one bit the time I called them about the APN settings. They had me take out the battery and SIM card like 10 times! Come on... really? Anyway, for my area (outside of Raliegh, NC) the coverage and speeds were about the same, but ping was always worse on Straight Talk.
You take what you can get I guess. I was happy with T-Mobile so I just paid for the $50/mo plan and deal with the 100MB of 4G data (unlimited 2G afterwards) since I'm around Wifi nearly 95% of the places I go. In fact, most recently (like last week or so) I jumped back onto my parents family plan on T-Mobile and just pay them the $20 for data and $5 added line charge. But as soon as I need to leave that plan I'll most likely be on T-Mobile prepaid, or ST (if the coverage for TM is just too poor).