I'd rank them like this:
Sears Tele-Games version - it has the detachable controllers. Compatible with INTV2 controllers
Orig version
INTV 2
You nailed it with the comparison to the Atari 2600jr. The INTV2 is smaller and looks nice. In addition to the limitations others mentioned, it also has a horrible on/off/reset combo button (truly awful), and that gigantic external power brick will cover several plugs on a power strip.
BUT the detachable controllers are a huge plus. If you have the Intellivision Flashback, you can buy a converter cable and use those controllers on the Sears or INTV2. They are a big improvement!
And if you get a LTO Flash (highly recommended), that eliminates the BIOS issue on INTV2 that locks out some coleco games.
FYI - I have a Sears unit and also an INTV2. The INTV2 with LTO Flash (and using a Flashback controller) has become my GO-TO system for Intellivision. Mostly because it doesn't take up much space.
One great thing about collecting for Intellivision in general -- the games are not expensive and plentiful. It has been a lot of fun for me.