LB is a bigger need than WR because we need play makers on defense one way or another. I wouldn't hesitate to take a guy like Davis at 30 or what have you if he is the BPA because we need another outside/vertical threat aside from Dez, but the fact remains that a rookie WR would be our 4th option in the passing game behind Dez, Beasley and Witten.
Of course Witten is on his way out and Dez will be 29 next year, that's why I would have no problem taking a WR.
In a theoretical situation where we have an evenly ranked LB and an evenly ranked WR, I'd go LB just because of the bigger need in general on that side of the ball. Lee will be 31 next year and it'd be nice if he didn't have to play 100% of the snaps, which could extend his career. Smith is a huge question mark and the rest is just depth. The impact of true three-down LB's is underrated, and while we may have a pair in Lee/Smith, they both come with significant question marks and it sure would be nice to not be forced into playing Lee on literally every snap.
It's true that we are often in nickel/dime and only have 2 LB's on the field, which is why I'd also be ok with taking a jumbo-safety type like a Deone Bucannon or Su'a Cravens in the 2nd-4th area.
To be clear, neither position is a massive need and if we don't take one in the first 3 rounds I'd be fine with it (assuming we are actually taking useful positions, not dicking around with TE's), but I could easily argue that taking either a WR or a LB in the 1st provides value.