I'm more opinionated then knowledgeable!
But I have been around for a bit now
I'm not keen on recommending a specific paint, as everyone's experience will vary and individual guns plus specific conditions on the day can affect this
I'm more inclined to avoid commitment to any quantity too far in advance before you have had time to try preferences and have dealt with suppliers for a while. All the main suppliers that you see regularly at uk events know about handling paint and are reliable, so you can be confident that they will look after it and be helpful to you if there are issues. A supplier who is off site can be as supportive as anything, but if they are not there then there is nothing that can be done about a bad batch
For the ego & ion they will be much more gentle than an a5 and you can confidently go for the 'higher' ranges of paint. But this does not guarantee that they will be the most gentle of guns. Preferably avoid the bottom level ball for these guns. You could use an upper range such as 3* but a mid range can still be a good option though it may be the 'safe' option
Go by the descriptions of the grades and different brands are pretty similar in their targeting of grades and types of customer
'Low' grade, mid level 'recreational' and upper grades that go for fragility on impact and thick fills
At a big event there will be complaints of wiping or people not taking hits. A thick 'unwipable' fill won't stop people playing on a bit, of a marshal is near and comes over to check them there is no true penalty, people can always say they didn't notice and then walk
If a lesser grade is not so well made then be balls are less consistent, but unless you are using very cheap paint most these days will have a decent consistency unless it is a bad batch or affected by the environment.
If in doubt try to avoid pre-order and do a quick survey on the day for what is good, if happy for a decent paint but not too worried about perfection then play safe with mid range, if you fancy that bit 'better' try the 'upper' grades
http://www.tomahawkpaintballs.com/products.asp?series=&box=&first=
http://gisportz.com/public/gi-catalog-frontpage/index.html#page-2-3
http://lips-paintball.com/acatalog/GI_SPORTZ_68cal_Paintballs_Equipment.html
Personally I use various guns as my mood takes me, so sometimes I need to be conservative on my choice of paint, but ranges available at scenario events tend to be good anyway. I've never had paint that is highly fragile
I have team mates that will be more attentive to the right paint and by the time I am considering paint they have already looked at other people's choices and seen if there are issues with specific paints, next comes the quick chat with the retailers and the paint is decided on.