More, I say.
Attackers usually rely on pace. One hamstring/cruciate injury can often rob them of that (see Owen). Furthermore, many attacking players get found out early on, and rarely manage to maintain their form throughout their career (see SWP).
Top defenders don't rely on such variables; for defenders, it's strength, coolness, ability to command the team from the back, quality tackling. Usually such qualities don't dramatically tail off - if anything they improve with age.
Now sure, defenders don't score you the goals that win games, and never command the same transfer fees as attackers do, but as a long-term investment goes, Stones is worth far more over the next 10 years than Sterling ever could.