well, technically, they work exactly the same way. ftp, email, web pages, scripts, dns… there’s no difference in how a web site is run no matter what domain name it is.
however, you may prefer one over the other for some reason:
1)price is different, very different. cost of .com .net .org domains is US$7-15, while .com.sg domain name costs S$60-80. so if there’s no special reason, stick with .com names, it won’t hurt most likely.
2)where is your target market? unless you want really strong local identity, .com domain name is generally acceptable. if you run a shopping mall, or yours is neighbourhood community web site, you may want .sg domain names as people can tell immediately who you are.
3)you are singapore branch of an international company? then you may want to register .com.sg domain and put localized content.
4)short, crispy .com version domain is taken while .com.sg name is still available. nowadays almost all the short, eye catching, 1 or 2 word .com domain names are taken by someone, it’s really hard to get a good .com name. however, there are still many good .com.sg names available.
5).com domain name is open to anyone, .com.sg is available to singapore registered companies only.
the same applies to .net .org and other country domain names.