Aloha Cruzer! I have both and love them for different reasons. The Fenix E01 is smaller since it uses 1*AAA and isn't as bright as the Tank E07 that uses 1*AA. I keep the E01 permanently on my keychain. The tank E07 is one of my edc's. Another small torch that I really like is the ITP A3 EOS which is also a 1*AAA light but has 3 modes (low,med,high) unlike the 1 mode E01 and E07. Since you already have a Tank E07, why not try the Fenix E01 or the ITP A3 EOS. You can't go wrong with either one. Good luck.
The Tank is considerably brighter and not all that much bigger. It gets around an hour from an AA, whereas the Fenix will give you 11-12 hours of very much dimmer light from an AAA. I have both, and would probably keep the Tank in preference to the Fenix if I only got to keep one. The Fenix is likely to be more reliable as there is almost nothing to go wrong with it.
Depends on what else you have with you. I tend to have either spare lights or batteries or both. I'd probably go for something other than another E01 as I'm pretty confident it wouldn't break and how often do you need light for more than 11 hours at a time - the Trustfire F23 runs for about 10 minutes on high with a 10440, my brightest light runs for 23 minutes on 4 5Ah Cs - or several hours on 3 Ds.