Each pink star is 1/4 of a life. Also, reds are the better choice of UFO in my opinion, as they give more total resources, and you can't lose lives without using them.
I think that strongly depends on how well the player does at a given time in a stage. For one green UFO you get 1 1/3 bombs, one red UFO yields you 1/2 Extra Lives. Since it's hard to use that, I'm just going to use 6 summoned UFOs as a basis; that's 8 Bombs (green only) or 3 Extra Lives (red only). With the latter, you can, theoretically, get up to 9 Bombs. 6 actual Bombs, 3 times you can clear the screen by getting hit. After getting hit, you're invincible roughly as long as a Bomb lasts (at least with Reimu A). During most stages, that should be enough to survive any problematic part. During Boss fights, however, it becomes a matter of how well you can handle the pattern during which you got hit. If you can clear it after getting hit without being forced to use another Bomb, then that's pretty much one additional Bomb. If you can't, however, then the value of the Extra Live itself you gained by summoning the red UFO starts decreasing.
Furthermore, if you get hit prematurely without using all your Bombs, then the loss is much greater if you summoned a red UFO. Assuming you summoned two red UFOs, you effectively wasted two UFOs by getting hit with two Bombs in stock. If you summon two green UFOs, you're only in that situation if you actually crawled back from 0 Bombs to 2 Bombs with UFOs (though getting hit without using Bombs would just be bad in that situation). If you got the two Bombs on top of the two Bombs you started off with, losing an Extra Life without bombing won't be quite as bad as in the same situation after having summoned two red UFOs.
The overall gain from red UFOs and green UFOs is roughly the same, but the actual value depends a lot on the situation and on the player. If you never get hit with Bombs in stock, it hardly matters which ones you summon, but if you're prone to getting hit with Bombs in stock, green UFOs would probably be more helpful. Furthermore, green UFOs can help you immediately to survive longer since you get 1 1/3 Bombs out of them. With a red UFO, this would only be the case if you happened to be sitting at 2/4 or 3/4 and even then, you would just get hit where you were supposed to get hit anyway.
One last disadvantage would be the loss of Power that you have to deal with if you get hit. Effectively, the theoretical third Bomb gained from a red UFO comes at the price of 1.00 Power. Most of the time, it won't make much of a difference, but especially during patterns in Boss fights you have problems with, this can force you to use another Bomb since you spend more time dealing with the pattern, giving you more room for error. Again, that depends on the player.
However, I don't think it's too good an idea to keep only the gain from UFOs in mind. For my 1CC, I set up a route that would let me get a total of 8 Extra Lives and spent everything else on Bombs. Especially for weaker players that's a good option I think. Players who can deal with the patterns but like to play it safe, however, would see more benefit from red UFOs if they can deal with risky patterns. If you summon green UFOs, you pretty much want to avoid situations in which you're sitting there without Bombs, so I would recommend green UFOs to less skilled players and red ones to everyone else I think.
In the actual stages, however, you can't always pick which UFOs you want to summon. Sometimes, a certain UFO will just come screaming at you - summoning more UFOs is always a good idea since the UFO itself can, if summoned properly, itself equal two Bombs without even considering the items it drops.
The bottom line is, feel free to summon whichever UFO you prefer, but don't decrease the overall amount of UFOs summoned to summon maybe one more red UFO or one more green UFO - UFOs are also needed to clear otherwise difficult parts, never forget that.
