If your only looking at keeping it sealed for resale reasons then yes it's probably not worth it - mainly because of the price you've paid.
Here's a couple of my reasons why I'm keeping mine sealed:
1) I just can't justify opening something that I paid £80 for instead of waiting a little and opening a £20 version.
--- Does it even really matter what you do with the sealed Newt? Paying an extra $40 arguably $20 for the Newt in the two-pack isn't alot and once you've got that, it doesn't matter what you do with the sealed one. Keeping it sealed gives you options, including the one of not knowing what to do it---
2) getting the two-pack anyway. Don't need 2 Newts.
---You should probably keep this in mind, if your going to get the two-pack anyway then it's only normal to consider: should I wait and open that version? if you need 2 Newts, if the sdcc accessories are worth it.---
3) I have a sealed and open collection so if I only have 1 then it must be sealed.
--- I know you've said before that you like the look of a sealed collection, if you ever decided to start a sealed collection of just your favourites or just Aliens or just whatever you wanted then you'd already have a start by keeping your Newt sealed.---
4) it would piss off Yahzee
---only joking
Yahzee's cool---
I think ultimately having Newt sealed is about whether you'd be content anyway with the two-pack version. If you would I don't see much reason to open the sdcc version, unless you really cannot wait.
Look everyone has their own decision and everyone is right in that decision, just be sure of it and don't regret it.