I ASSUME that you can obtain a Cacturne first( i haven't got R/S tho, so my info is not confirmed) but the desert where you get cacnea and to evolve should come before you can get Tropius because Tropius is a big powerful pokemon that you probably can't get until late in the game whereas Cacnea is weaker so should be obtainable first.*Takes a breath* Basically i would hav thought u can get Cacturne first, and on the Cacturne or Tropius front... More people favour Cacturne cos of it's Spiking/sandstorming abilitities.