I recently got a set of bridge size desjgn (red & black) cards to add to my collection due to the what everyone says about them on this forum. After playing with bullets, KEMs, well as the desjgn cards I have few thoughts on them.
In terms of how stiff/firm they feel, bullets are the stiffest/firmest, dejgns are in the middle, and KEMs feel the softest/flexible. I find the stiffer/firmer cards are easier to riffle shuffle. The more flexible the cards, the more chance you'll grab 2 or more cards with the thumb as you pull the corner/edge of the card up to begin to layer them on top of each other. However, I'm assuming the benefit of more flexible cards is that they're less likely to crease/warp and keep to their original shape after a ton of use/shuffles. I guess this is why KEMs have a reputation for being durable/holding up after years of use.
In term of how slippery/smooth the backs of the cards are, bullets are the most smooth, desjgns are in the middle, and KEMs are less smooth/rougher (don't confuse this with a bad feel). I find the bullets are the easiest to push together during a riffle shuffle, desjgns again are in the middle, & the KEM cards take more effort to push together. However, this smoother back of the card also leads to a higher likelihood of the top card sliding off the top of the deck after being put down. If you're playing surface isn't level, you may find that to be a problem.
With these differences in mind, do I think you should consider getting a set of bullets if you already have a set of KEMs or Desjgns? Not really, unless you find that you really want a deck that is firmer/smoother compared to KEMs. If you have a set of desjgns, the differences are small but noticeable.
After using a set of Desjgn cards, I can see why users on this forum like them so much. They're very much the Goldilocks of cards in terms of how they feel. Desjgns are right in the middle of stiffness & smoothness. Considering they cost less than KEMs/bullets AND include a cut card (when being shipped in the US), i'd recommend them to anyone looking for their first set of plastic cards. I find it ridiculous now that sets of premiums cards costing over $20 don't include a plastic cut card.
If you're the one shuffling/dealing at your games and you want firmer cards that are easy to shuffle & deal I can recommend the bullets in that case. Just don't forget your cut card.