I've got a 510 on one side, and a TiVo standalone with a 301 on the other side.
Benefits of TiVo SA + 301:
- Better remote
- Name-based recording
- If you pay unit "lifetime" fee up front, no monthly fees
- Can seamlessly record from OTA antenna (510 requires sub for locals)
- TiVo Series II can serve up audio, photos, and much more as part of in-home data network
- Otherwise unused hard drive space gets filled with sometimes-useful "suggestions"
- If you ever leave Dish, TiVo can be used with cable or D*
Benefits of 510:
- Better integrated guide (TiVo can miss new channels for a few days)
- Better picture quality (TiVo must recompress video to record)
- 100% channel accuracy (TiVo IR to 301 makes mistakes now and then)
- Cheaper monthly fee
- TiVo gets stuck on some "You Are Not Authorized" screens that require channel up-down to exit (blacked-out event, for example).
- One less device to plug in and manage
My 510 has a lot more hard drive space than my TiVo, but that can vary according to model.
Over the year+ that we've had both, we've migrated to the 510. When the TiVo fails to change channels to a desired program for whatever reason, that's no fun. Add in the better picture quality and (for us) increased storage, and that's why our TiVo is now our secondary DVR. For now.