12 Mbps is not comparable to 500 Mbps, the ATT rep was just trying to make a sale. 12 Mbps service will be much slower than the 400-500Mbps you are getting now, I wouldn't switch. If you can only get 12Mbps with ATT Uverse, it is probably a DSL connection, while your Mediacom Xtream is likely fiber, much more stable technology. If the bill is too high you can always downgrade to a slower tier with Mediacom I imagine.
Keep in mind that with all residential internet connections, the quoted speeds are "up to" a specific number. This can only be reached when there is no congestion, as the ISP's available bandwidth is oversubscribed (typically 20+ times). When more customers get online in peak hours and start video streaming, it usually slows down as they have to share the available bandwidth. For example, your "up to" 1Gbps (1000 Mbps) connection is giving you typically 400-500 Mbps service. A DSL connection that is "up to" 12 Mbps will likely max out at 80% of that.
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