There is no marketing "standard" storage capacity. A 160GB drive can store about 35 hours of Universal HD, 25 hours of FOOD-HD, 19.5 hours of ESPN-HD, or 18 hours of a full-bitrate ATSC channel. Which figure do you use when marketing a DVR?
Actual capacity will be a little less, because DVRs also reserve some space for their software as well as the HD buffers. Different DVRs may reserve more space for the buffer than others.
TiVo's claimed 180 hours for SD are for analog channels at the lowest quality setting. With FiOS and CableCards, there are no analog channels, only digital channels that are recorded as-is. TiVo capacity with FiOS SD will vary from 80 to 120 hours depending on what SD channels and content you are recording.
The Motorola DVR doesn't allow drive upgrades, but you can easily upgrade the 160Gb drive in the TivoHD with a $100 500Gb drive or a $250 1TB drive. Here's a screenshot of my TiVo with a Western Digital 1TB: