If cash is your limiting factor i.e. it's a choice between being able to afford one or the other...... then hoppers.
That way you have a unit that can cope with anything that's put on the board.
Buy ripplers and he fields an army of exos or marauders.... wasted money.
If you've got the cash, get both. Problem then is knowing what your opponent is fielding. If he only has infantry models and no exos, chas or marauders then ripplers are the way to go. What happens on the day he turns up with an army of marauders he bought cheap from Ebay? Then again, I'm sure if you shop around you could do the same.
The game plays bettter when you don't know what your opponent will be using (a game is only balanced when neither army is built knowing what the other will play)...... unfortunately, most wargames can't take into account that extra bit of 'intelligence' you get from knowing what's sitting on your mate's shelf
I use cheap workers etc.... for taking out the infantry. Keeping the hoppers back for exos etc. Using a unit of hoppers can cripple a unit of exos quite easily.
E.g. Opponent dropped an exo platoon on my side of the board, COugar LT, Grizzly NCO and two units (3 with missiles, 3 with sixguns). I flew in 20 hoppers in two units... first unit took out his LT and two of the sixguns, he then had reaction fire from the remaining sixguns (two in the unit, one on the NCO). Second unit came in and took the remaining sixgun and grizzly sergeant. I then coordinated to take the NCO.
Left two grizzlies with missiles... no command, pack weapons so can't react.
Hoppers are great for this and exos are really susceptible to losing their command structure... most players will double up the missile launchers to get the extra penetration/multihit effect, that means you only need to take out the sixguns to stop reaction, then use coordinate to get a second attack and take out the command models (exos have two hits, so they can't 'substitute' a lesser model like other MI).
But Adastra is right, if you can afford it, buy both, it keeps an MI player guessing if you have both on your shelf (planning to cope with both costs points and prevents cheesy armies). If it's one or the other, go hoppers, they can put a hole in anything.