Testing latency to different regions is obviously advisable! I'm located in Australia and many users here get better latency to US West than to Singapore - in part it comes down to local ISPs peering and international connectivity. It's relatively straightforward to test if you have users in the region you're targeting.
Reliability on the AWS side (i.e. not user network issues) is mostly a consequence of deployment across multiple Availability Zones. There are more choices in US regions than in APAC ones simply because they've been serving those markets longer. A side effect of this is that features are deployed relatively late to Singapore/Tokyo - normally new features start rollout in US East.
As you already have S3 and EC2 in mind as services you'd like to use and they're both available in closer regions, evaluate whether newer web services from AWS are immediately important - if not, shoot for something (latency) close by.
EDIT: Look at Mark Tsai's answer. That's the way to go (Route 53 didn't exist when I wrote this one)
This probably belongs in ServerFault but here goes:
What you are basically asking for is Geo DNS.
Right now it's not directly supported in AWS - although I've seen some talk of it being implemented in some AWS forum posts - most probably in their Route 53 service.
Until then, you could look into 3rd party solutions such as Zerigo that would provide you with Geo DNS facility.
As for your question regarding performance and AWS reliability: You should consider serving up your site from the nearest AZ to your user - it makes perfect sense in terms of speed and not having all your instances in a single AZ. You could check AWS Service Health Dashboard to get an general idea how reliable Amazon's services are in different AZs. Note that this data is directly from Amazon - I've not seen any independent stats anywhere else.
This project is used to generate the indexes (and visual map for
reference) used by the TurnKey Hub to find the closest AWS data center
for a user.[emphasis mine]
While a bit of a gimmick, the visualization is seriously cool and confirms resp. illustrates the reason for the at first sight surprising fact Josh mentioned already, namely that users in Australia currently tend to get better latency via the US West (Northern California / us-west-1) rather than the Asia Pacific (Singapore / ap-southeast-1) region. (Tip: checking Future Cables at the bottom right corner reveals this is likely going to change, which is further detailed in Greg's Cable Map, which indicates Australia might jump between both AWS locations latency wise in the years to come ;)
Using lowest latency AWS location automatically via Amazon Route 53
Meanwhile AWS is providing a helpful map illustrating their Global Infrastructure for a quick assessment, alongside respective details like e.g. number of availability zones and the API endpoint.
So in case your environment is comprised of an Auto Scaling EC2 Instances architecture already, simply applying this latency based routing should solve your problem automatically.
While the use case obviously targets offerings spawning multiple AWS regions, the sophisticated features around Latency Based Routing and Weighted Round Robin Record Sets might allow you to determine the desired information more easily yourself as well.
Try cloudping.info, it will do HTTPS ping from your browser to each AWS region.
Region Latency
US-East (Virginia) 28 ms
US-West (California) 100 ms
US-West (Oregon) 110 ms
Europe (Ireland) 100 ms
Europe (Frankfurt) 119 ms
Asia Pacific (Singapore) 269 ms
Asia Pacific (Sydney) 239 ms
Asia Pacific (Japan) 209 ms
South America (Brazil) 147 ms
➥ ./awsping --verbose 1
Code Region Latency
0 eu-central-1 Europe (Frankfurt) 36.97 ms
1 eu-west-1 Europe (Ireland) 63.18 ms
2 us-east-1 US-East (Virginia) 126.52 ms
3 ap-south-1 Asia Pacific (Mumbai) 156.98 ms
4 us-west-1 US-West (California) 192.92 ms
5 us-west-2 US-West (Oregon) 226.23 ms
6 sa-east-1 South America (São Paulo) 247.74 ms
7 ap-northeast-1 Asia Pacific (Tokyo) 312.22 ms
8 ap-northeast-2 Asia Pacific (Seoul) 329.54 ms
9 ap-southeast-2 Asia Pacific (Sydney) 337.84 ms
10 ap-southeast-1 Asia Pacific (Singapore) 395.73 ms
Regions are ordered by the latency.
You can run it on any server and determine the nearest region for you.
http://blog.datapath.io/aws-network-latency-map discusses a commercial offering to get this information. It shows latency time from the location you specify, to the AWS service you specify, on a map.