Yes, the TiVo service is software driven, but there are some physical differences between the units. As you can see by my signature below, I have 2 units, one Sony SVR-3000 and one TiVo branded unit. Although Sony no longer manufacturers any TiVo products you may be able to find one on eBay, or Craig's list, etc. I mention this for 2 reasons. First, the remote on the Sony is superior by far. Second, the chip used to decode the cable signal seems to be of higher quality in my Sony unit.
First, the remote. The buttons are well placed and respond well to the touch. They are in a logical place. Without moving my hand, I can pretty much access all of the important buttons. However, on the TiVo remote, in order to change channels, I need to slide my hand all the way to the bottom, punch the numbers, then find the micro "enter" button. Also, the Sony has different buttons for up, down, left, right. This avoids some "fat finger" confusion that you may find on the TiVo branded remote, where all of those functions are on 1 large button, and the remote responds based on the side of the button you push. The Sony remote is certainly a better design in my opinion. Additionally, the Sony unit has buttons on the front of the unit, to control all the functions (in case you lose the remote), while the TiVo unit requires the remote since it has no buttons on the front of the unit.
Second, the chip used for processing the incoming cable signal seems to be better on the Sony. Each of my TiVo's are on different TV's (of similar quality) in different rooms. The Sony TiVo produces a better picture that the TiVo unit. In the past, more technically sophisticated people have told me that this is due to the chip. I do not know if this is actually true or not, but it does make some sense considering Sony has the reputation for high quality products.
Whatever route you eventually go, you should be able to upgrade to a larger hard drive. Also, the Sony unit that I am referring to is the SVR-3000.