Welcome to the forum. Check out this thread for more details about TiVo HD's. Check out this post for spec comparisons between the Series3 and the Tivo HD. To get more recording space you can add an external drive via plug and play. Additional options for increasing recording capacity can be found on this thread. We have a Series3 and a TiVo HD and both are very good. If I were to buy another it would probably be a TiVo HD based on price, the fact that they have stopped making the Series3 and that TiVo HD's appear to be better situated architecturally for the future IMHO.