Tavares proved after signing in Toronto that his numbers weren't likely to skyrocket just by having better wingers. Playing with an elite playmaking winger (Marner) and playing on a PP with an elite goal scorer (Matthews) still only resulted in 88 points.
Ultimately, they're probably on the same sort of tier. But I'd give the edge to Spezza because of actual results.
Tavares' 5v5 production actually significantly improved his first year playing with Marner. He scored 21 more 5v5 points in 18-19 than he did in 17-18 going from 39 points to 60, which also beat his career best by 12. The issue was that his PP production suffered because Toronto had a lack of PPs and Babcock split PP ice time. His PP production fell from 30 points to 19, with his minutes falling 30 seconds a game. If he played his regular PP minutes, he would have been around 25 PP points, which would have pushed him up to 94 total and a 10 point increase over the year before. Obviously projections aren't perfect, but I don't think he's shown at all that he wouldn't have an increase with better linemates. Spezza still played huge PP minutes those years along with the 2 stars at 5v5.
Spezza also played with 2 elite wingers not just one while Tavares and Marner played with Hyman who can't create much on his own. I don't believe secondary assists are worth nothing, but if you're playing with 2 other elite players it makes it easier to collect secondary assists. 27.5% of Spezza's points from 05-06 to 07-08 were secondaries. In comparison, only 18% of Tavares' points in 18-19 were on secondaries and 21% in 14-15.
These things don't have the huge effects some people think, but they add up, and it's not like their scoring relative to the league was significantly different.