I bought both and I really like the Acroball better. They are very similar, and either would probably satisfy the casual user just fine. But, since I had the chance to compare, I noticed some differences worth noting. Per ink and writing dynamics (the "ball" and flow), they are both very good, smooth, consistent, nice! Both inks seem to dry equally fast and both seem to be resistant to water (although Uni makes claims and Pilot doesn't seem to, that I could find). Really, the only difference in the inks (that I could tell) is the flavor of the hues. The Jetstream has a more austere, cold, "serious" set of colors (that I actually prefer), and maybe more "opaque". The Acroball has happy, fun, warm colors (and maybe a little more "translucent"). But, yeah, they both rock on making nice lines with a smooth flow; neither skips nor falters (nor dry-starts). BUUUT!. . . .. after checking out the Acroball 4, and then picking up the Jetstream 4, I was shocked (shocked!, I tell you! :>) at how much less ink is in the Jetstream. First thing I did was check the seller's picture, and sure enough, that one was equally low on the ink. So, as best as I can tell, the Jetstream (which cost me $2 more) has about 3/4 or 5/7ths the amount of ink as the Acroball 4. So, not good. Acro refills have about 7.5cm of ink (length) and Jetstream is ~5.5cm. (note the positioning/height of the clickers on the pen bodies. . . . and then there's a much larger air space at the tops of the Jetstream refills.) Refills are 3 and 3.2mm diameter, respectively; but do the math, and that doesn't make up for it. The "click" (i.e. engaging a color and changing colors): So, the Acroball has a nice, solid click mechanism that works reliably every time I go to use it. It just works, and it feels good working it. The Jetstream has a more anemic feeling mechanism which doesn't always "take". So, I can casually swipe at the Acro, and it always does just what I want. The Jetstream sometimes gives up and pops back (retreats). Mind you, the Acro feels easy, smooth, light touch . . .. but just right and not anemic (weak). The "grip": User's preference here. They are about the same size. Jetstream is uniform 12mm diam. Acro has a waist and the narrow part is just under 12mm and the wider part gets up to about 13mm. The grabiness of the rubber is stronger on the Acro. I actually prefer the less grabby Jetstream in that respect. I use tools, and grabby rubber grips give you blisters after a while; whereas smooth wood or fiberglass handles (that slide a bit in your hand) end up being the better choice for prolonged tool use. But hey, it's a pen, not a hatchet. Short term, immediate user impression, the Acro feels better. Uniform cylinders feel cheap. The "clip": I like the beefy, spring-loaded clip on the Acro. It doesn't really "bite" . . .. it's pretty smooth on the part that would bite; but the spring is strong and it opens real wide. The Jetstream clip is fixed and just relys on the maleability of plastic to do its flexing. (I have visions of the Jetstream clip breaking with time.) The Acro clip says, "I got this". The Jetsream clip says, "I'm cheap". Still, they both work fine . . . really. Oh! . . . to add . . . the Acro does not have a nub/lump/bump on the pen body under the end of the clip (hence, the no bite). The Jetstream has the bump/nub both at the end/underside of the clip, and one just behind that on the body (so, "bite"). Really, tho, had I not had them both to compare and rabbit-hole this non-issue, either of these pens would have been just fine, and I would've been happy with my purchase. (but the skimping on the ink thing . . . . hmmmm)
