ACT has been around a very long time and has a proven track record across several MMO's. That is where it gets the reputation of being more accurate. It does have way more options due to it being around for so long. But there is a reason why it is the goto parser for most of the larger raiding guilds that migrated from EQ2 and WoW. They trust it's values and have independently verified it.
The parser from Riftjunkies gives you what ACT does in a much more simplified fashion. It's easy to use and is reasonably accurate. It is true that there have been complaints about accuracy issues due to how it and when it calculates fight duration times. It's a relatively small variation + or - 30 DPS or so, but there is a variance there. But if you just want a good indicator of what's going on then it'll do the job just fine.
I've used both in Alpha and now in Live. I currently use ACT because I prefer accuracy over ease of use. And because I like to numbers crunch and theorycraft lol. But I didn't have much of a problem with the parser from Riftjunkies either.
And Charlatan is correct when you say it depends how it's being used for. I'm a healer and since I'm not interested in HPS or DPS, then either parser is accurate. Same goes for Tanks who are measuring incoming damage taken. It's when you need to measure actual DPS that becomes a problem with Riftjunkies.
Also, while ACT is very complex, you can set it up very easily with a small tutorial. And later on, it gives you much more data at your disposal. If you have questions about it, I've played around with it and can help you out.