The 30 round springs can be purchased at a better price than you have listed. CMMG has spring and new green follower for $7.95 and I have purchased them from the Computerguy on the EE for 4.50 a spring. The price you have listed at Brownells for the 20rd springs is the best price I have noted.
At $4.50 a spring its pretty good insurance, the choice will ultimately depend on what you use the mags for, competition and/or self-defense or merely plinking at the range. It would also depend on the use your current mags have seen, new mags with the old type springs may not be worth the extra purchase.