I have both of those bags and love them dearly. I can't see myself selling them both. I would love to have a trevi but I agree it may not be an everyday bag and have heard it is fairly heavy. If you think that you will just never get past the fright of carrying the tivoli I would sell it and put that money towards the trevi and save for the rest while keeping the NF.
spongie08- I have a Tivoli PM and a Neverful Damier Ebene as well.... AND I'm in love with a Trevi PM which I have yet to get, so I completely understand your dilemma. It's been raining a lot lately so the Tivoli has been in the closet for some time. The Neverfull comes out when I've got loads of stuff to bring or when it's raining, and I use my Solar Mahina when it might rain. I'd like a Trevi as another everyday bag. It's currently out of stock but it works in my favor since it gives me time to save.
Regarding your dilemma, I'd keep both Tivoli and Neverfull and save for a Trevi. On sunny days the Tivoli is great! And the Neverfull is great for those days when you need to take everything but the kitchen sink. The Trevi will complement your bags because you can use it even on rainy days. Plus, if you sell, you'll lose some money and you'll have one bag the Trevi. Let me know what you decide!
I would keep the two bags you have. I wanted the trevi so badly, bought it and sold it shortly thereafter...it just wasn't for me I love the neverfull and don't think I will ever sell mine!! Good luck deciding