Alan Hahn and Mike from Knickerblogger have been talking about Ariza vs. Chandler.

Their same age stats are included here: [Only registered and activated users can see links. ]

I would take Chandler over Ariza.

Ariza tends to take higher quality shots. That accounts for the higher FG%. He also gets to the free throw line more often. Both those things tend to help his overall shooting efficiency (TS%). However, overall, Chandler is the more productive offensive player. I think that makes them about a wash offensively. Their rebounds, assists, blks, steals, and tovs are also close to a wash, with each player having an edge in different areas.
The reason I would take Chandler over Ariza is that Ariza had way more NBA experience at the same age as Chandler and has looked like a more seasoned player than Chandler does now for a few seasons. He actually has not improved all that much at all in the last few years (other than his outside shot).

Look at Ariza's Per 36 minute stats each year here: [Only registered and activated users can see links. ]

Chandler is still basically a rookie with many easily identifiable areas of improvement that are likely to occur if he makes any effort at all in the off season (he did improve in many areas this year over his limited season last year).

Look at the improvement in FT%, eFG%, TS%, PTs per 36 etc.. here: [Only registered and activated users can see links. ]

His only significantly declining area was rebounds Per 36, but he showed huge improvement in March and April when D'Antoni asked him to rebound more.
Also, even though playing with superstars like Kobe, Gasol, etc.. only helps a role player at the margin (small enough to be
difficult to seperate from other factors), it's obvious that great players draw double teams. That has to make it easier for their teammates by getting them an occasional wide open shot (just ask Ariza if he likes playing with Kobe - rotflmao). It's likely that Ariza's improved outside shot for LA these days (which is still worse than Chandler's) is at least "partially" the result of getting better looks because of Kobe. If Chandler was getting those same high quality looks slightly more often, his shooting stats would also improve at the margin.

Finally, If Chandler starts getting to the free throw line more often by being consistently aggressive, there will be no comparison between the two because Ariza is a fairly weak FT shooter and Chandler is becoming a very good one. He made huge strides this year over last year.

On defense, Ariza has a very good reputation (tough to prove how good statistically), but Chandler is no slouch either - being able to guard 3 positions and also improving from last year.