Trouba's value IMO is much higher than Spooner because of his pedigree, draft position, and ELC performance at ages 19-21. Lest we forget, Spooner + 2nd wasn't enough for an impending Chris Stewart last deadline.
Push comes to shove, I'd be willing to give up Eriksson + the lowest of the B's 2016 1st round picks for Trouba, plus another prospect... it would be really tough to include Subban but if that put the B's over the top and they got to retain their highest draft pick, I'd pull the trigger. In that case, they'd probably try to get an extra body like Mathieu Perreault (healthy scratch tonight apparently) back in the deal to offset the short-term loss of Eriksson.
I've gone back and forth over which of Shattenkirk/Trouba I'd prefer - I think the fit for Shattenkirk is more apparent as Hitchcock and Armstrong both seem like they'd love Eriksson, and he'd be the rental piece they'd need to compete with Chicago. He and Lehtera seem like they'd be a seamless fit, and Schwartz is due back shortly after the deadline to complete that second line.
Shattenkirk is also more perilous in terms of what he'll command on his next deal, with only one year at $4.25m AAV remaining and UFA impending after that. Trouba's ELC expires this summer, and with his skillset I have a tough time imagining he'd get more than ~$4-4.5m on his second contract.
All that considered, and with Chara's career winding down... I'd go all in for Trouba.