Short answer: if you're not planning to use it live, go with the EPSi.
Long answer: both are "just" IR loaders/players. This means they don't have a "sound" -- they sound as good as the IR you load into them. However, the EPSi is a simple version of such an IR loader, it can only be switched manually (turn the dial and hit "recall" to get another IR). It can load a lot of IRs from an SD card but you can't switch it e.g. coordinated with the 11R. So if you want an IR of 2x12" open back cab for your Fender Twin sound and a classical 4x12" for your Marshall sound, you'll have to switch it manually. Not suitable for live.
It does have some parameters to tweak the sound of the IRs (simple internal eq).
The Torpedo Cab is a more complex unit. You can save 512 IRs, you get a 5-band eq switchable for guitar or bass frequencies, headphone out and (most important) MIDI in. This makes the CAB switchable in conjunction with the 11R. Simply put your Fender cab on the slot that corresponds with the MIDI program change that calls your Fender Twin, and the 4x12" on the slot that corresponds with your Marshall.
When do you need this? Live. What do you do in the studio? You switch manually anyway - or you can do so at least.
I am pretty happy with one of the factory IRs in the EPSi which works equally well for clean and overdrive sounds, e.g. all types of amp sims from the 11R. So I decided not to go for the Two Notes which is 2,5 times as expensive and will basically sound identical apart from the EQ (wich, I think, you don't really need, the 11R has a good one on board).