I've got one of each...
1. The Sony SVR-2000 standalone (Series 1) is my older TiVo, and works with an even older RCA DirecTV satellite receiver. It's got Turbonet, 120 GB hard drive, telnet, ftp, tytool, and TivoWeb. It's definitely easier to hack a Series 1 (in my humble opinion).
2. The Philips DSR7000 DirecTiVo (Series 2) is about a month old, and totally unhacked. The kernel monte and or kernel/userland swap tricks work, but seem like too much effort to me. I picked this DTiVo up for $99 on DirecTV's 'ffdvr' promo to replace an older RCA satellite receiver that was flaking out way too much. The only way I'm going to hack this one is to send the motherboard out for a PROM hack, and whack the initrd. Between the stock PROM and the initrd, the Series 2 TiVo cleans out any and all hacks / patches everytime it boots.
3. Standalone Series 1 TiVo's have a slower 50 MHz PowerPC processor, but also have a dedicated MPEG encoder chip, that offloads the hard work from the PPC. They also have an edge connector on the motherboard for your TurboNet adapter.
4. Series 2 TiVo's don't have dedicated MPEG hardware, so they use a more powerful (200 MHz?) MIPS processor to do *all* of the work. They also have USB ports to connect an Ethernet dongle/adapter. It won't work until you hack the box to get Bash, and FTP over the kernel modules, or until you purchase HMO (Home Media Option) for $99 from TiVo.
My personal suggestion is:
a) purchase one of the many Series 1 standalone TiVo chassis off of EBay
b) purchase your own inexpensive 80 or 120 GB hard disk drive (BestBuy has Seagate 120 GB for $149 less $70 rebate = $80 !!)
c) download a TiVo backup image from one of the many FTP or web pages listed here and elsewhere, restore using MFStools.
d) install and configure your TiVoNET or TurboNET adapter.
Series 1 SVR-2000 w/TurboNet