SP from injectors is cheaper than SP from booster packs when comparing SP/PLEX ratio.
(*) With injectors you can max out your character’s SP (566’016’000 SP) with “just” a ~€22k, with booster packs it costs ~€2,6mil, so the cost ratio is almost 1:10 in favor of injectors at current PLEX prices.
It is possible to buy infinite number of packs per character if you transfer your character on a new account several times with additional cost of character transfer and buying omega for each account if you want more than 20mil SP.
(*) the math is here but the booster skill packs names might be non existent now as I noticed recently CCP replaces them regurally. math was valid at the time of the post 