A Mobile Virtual Network Operator (MVNO) is nearly always going to be cheaper than the 4 major carriers whose networks they use, T-mobile, AT&T, Verizon, and Sprint (with the possible exception of cheap multi-line family plans). The main reason MVNOs are cheaper is because they don’t have to maintain stores, build cell towers, purchase spectrum, etc. The major networks benefit because they lease to the MVNOs and get paid for customers they might not normally get.
Most MVNOs these days are as fast as their underlying major networks. Occasionally, if networks are very congested, the major carriers will get higher speed data while the MVNOs’ customers are temporarily de-prioritized (with slower data), but this is rarely a big problem. Also, MVNOs may not have roaming agreements with other carriers, like the majors do. Coverage everywhere is generally getting better over time, though, for all networks.
I know T-mobile gives out some perks and freebies to its customers with certain plans, so that’s out, too. (But you will probably save on the overall price of service with an MVNO.)
As for MVNO customer service, it can be hit or miss, depending on how well-established the company is, and is usually conducted by online chat from your account, or by phone. There’s no shop to walk into and no salesperson to hand your phone to, but ask yourself how often you really need customer service after you’ve initially set up the phone. For me, I insert a SIM card, the phone downloads the appropriate APN settings, and I’m off and running, usually.
I’m someone who typically uses very few minutes and very little data per month, so I naturally look for the cheapest options that will give me good coverage at the right price. (My current monthly plan is $10+$1.48 tax/fees for Unlimited/Unlimited/1GB LTE.) I also buy my phones carrier-unlocked, so I have the freedom to switch when I want.
Red Pocket* is kind of unique in that they have plans on all 4 major networks, so you can choose a SIM that works on the network that gives you the best coverage and works with your particular device. It’s also easy to switch to a different one without leaving Red Pocket, just by ordering a new SIM and chatting with customer service. But there are many good MVNOs out there, and others sometimes offer a unique benefit that fits your needs.
A good site for independent reviews is: bestmvno.com which is run by a guy who actually tests them for a few weeks at a time, then reports on his experience. You can filter the site by AT&T or T-mobile (or Verizon or Sprint) MVNOs to find exactly what you’re looking for. Then always visit the MVNO’s main site to make sure you’re seeing the latest plan configurations and prices. You can usually order a SIM card from the MVNO’s site, or sometimes from Amazon or eBay, or pick one up at Best Buy, Target, or Walmart.
*Edit: I’ll add that I’m not endorsing Red Pocket as the only or best choice. It all depends on your needs.