(New! list of deaths, from HP wiki. Analysis at the end.)
I'm going to say that it was generally safer to be a Death Eater because the good guys will at least try to capture them, while the Death Eaters won't hesitate to kill. Consider the three groups fighting:
First, the Order of the Phoenix. They were probably the most powerful of the groups, but also the most principled. I'm pretty certain they didn't kill the death eaters they fought unless absolutely necessary.
Second, the Aurors (not all of whom were OotP, I'm quite certain). At the beginning of the war, they weren't allowed to use the unforgiveable curses, and they were supposed to capture rather than kill. Eventually they were given the authority to kill, but presumably they still captured when it was practical to.
And thirdly, the Death Eaters. They were, as I'm sure we all know, ruthless. They had no compunction about killing, and I would venture that at least those fairly high up the ranks were at least as powerful as the Aurors, and probably even as powerful as some in the Order. That makes them quite dangerous. Voldemort himself, while fond of torturing his followers, only actually killed them on rare occasions (e.g. hearing about Hufflepuff's cup, he didn't want anyone else to know about it, and the news put him in a murderous range, so he just killed anyone in the room he could).
So from a practical standpoint, the weaker Death Eaters were most likely to be captured, not killed, by either the Order or the Aurors, and the stronger ones were less likely to be either captured or killed because of their skill (though probably more likely killed than captured).
The Aurors and Order, however, would almost certainly be killed if they were defeated.
So, for a quick calculation, let's say the Order primarily fought the stronger DEs and had a 2/3 probability of beating them, of which 2/3 of the time they'd capture them alive. That would result in 1/3 (3/9) of the time, the Order member would die, 2/9 the DE would die, and 4/9 the DE would be captured. That means that even though the Order member has a better chance of winning, they have a higher probability of dieing. Let's say the probabilities are similar for the Aurors vs. weaker DEs, and say 9/10 of the time a stronger opponent (OotP/Strong DE) would beat a weaker one (Auror/weak DE), with the OotP always capturing and the DEs always killing. The exact final ratio would depend on how often the strong wizards faced strong vs weak DEs, and vice-versa, but in all cases, it seems the Good Guys are more likely to die.
So, unless you count life in Azkaban, it seems the Death Eater's side would probably be safer.
Now for some absolute numbers described in the book:
OotP DE Other Muggles
First War | 10 | 3-4 | 1 | 12
Second War | 8-11 | 5+-8+ | 15 | 2
Total (approx) | 20 | 15 | 16 | 14
So this shows a higher absolute number dead for the Order, and considering that there were slightly more Death Eaters than OotP (from another post I can't find at the moment), their rate of death is certainly higher. However, there is a fairly strong reporting bias (especially in the first war), so it probably wasn't quite as lopsided as it seems. Interesting to note, however, that not fighting in the war was also fairly dangerous. (And my 1/3 vs 2/9 = 3:2 estimate isn't terribly far off for the second war).
The actual list. Asterisks (*) indicate the ones I'm not quite sure should be counted in that particular category or at all.
OotP: 18 - 21
- (First war) - 10
James Potter (V)
Lily Potter (V)
- (Second war) - 8-11
Death Eaters: 8+ - 12+
- (First war) - 3-4
- (Second war) - 5+-8+
Quirrell* (Harry - sort of)
Barty Crouch Jr* (Dementors)
Several (V's fit of rage after hearing about the cup)
Misc. wizards: 16
- (First war) - 1
[Dean Thomas's Father]
- (Second war) - 15
Bertha Jorkins (V)
Barty Crouch Sr (BC Jr)
Cedric (Peter Pettigrew)
- (First war) - 12
12 (Peter Pettigrew)
- (Second war) 2
Frank Bryce (V)
German Woman (V)
Muggle Tramp (Locket horcrux)
Albanian Peasant (Diadem horcrux)