Don't forget that Shane Watson was our first choice for the Test side this summer. In Test cricket I would pick Watson because his batting seems more solid than Symonds. He has made more runs at a better average at first class level and is better technically than Symonds. He would be playing only as a 5th bowling option, so even if you aren't convinced Watson's bowling is any good, it wouldn't matter much. Batting is the most important attribute for the #6 not his bowling.
In ODIs, I'd pick Symonds first. Mainly because he scores quicker, is better in the field and his bowling is reasonable enough to fill in 5-7 overs, 10 if the conditions suit his off spinners. I don't really rate his medium pacers, especially if he has to bowl under pressure late in the innings. He really needs friendly, swinging conditions to bowl the mediums well. Trouble is, Mike Hussey bowls them nearly as well as Symonds.
But I must say that Symonds is NOT as good as people think he is. Somehow he has this reputation of being a great player and recently more than a couple have said he would be in any World XI. But really his batting is just average to good but fans think it is much better cause he hits a few sixes here and there, much like Adam Gilchrist. Give me Ricky Ponting or Mike Hussey any day.
And his bowling is pretty toothless really - but it does suit ODI cricket pretty well as some players find it hard to go after, like Jayasuriya or Chris Harris. His bowling reputation has enhanced cause he is "so versatile", able to bowl pace and spin, but it doesn't matter how many different types of balls you can bowl if none of them are any good. The only area where he is world class is his fielding, something Bill Lawry will tell us about 1000 times every summer.
So there you have it. My opinion - Watson for Tests, Symonds for ODIs. And my vote goes to Watson, as tests are more important and Watson has the potential to be better at ODIs as well, if he gets his bowling sorted and he gets selected as an opener.