You have a bit of a dilemma but not one that cannot be resolved.
On the one hand you have a domain with an established trust score, site rank, and various pages with rank and links to them you do not want to lose of course. But that domain name has limitations due to the ccTLD and how they would propagate out into the search engines.
The first question is: Where do you want to or need to do business?
With the Singaporean .sg ccTLD, your reach would be limited to those Google websites that have a history where a .sg ccTLD has performed well. This would be based upon locale and language primarily and the likelihood that someone in x country would be interested in your site.
If you need to expand beyond this limitation, then the .com domain would present opportunities to expand your reach into other countries. But this domain does not have the advantages that the .sg domain has due to age and performance over the years.
It is not uncommon that people change domain names. However, there is always a price for this. Here is what is typically prescribed.
- Move the old domain to the new one.
- Create a 301 redirect to the new domain.
- Examine the link profile of the old domain.
- Create a comparable link profile for the new domain.
- _That can mean having links changed or creating new ones of similar value.
What is the cost of moving a domain?
- Losing the trust score attained by the original domain.
- Losing the site rank of the original domain.
- Loss of a small amount of rank passed using a 301 redirect.
How to recover some of this loss.
Establish a new trust score by:
- Creating a location/locale schema.org mark-up on the new site that is
easily found. This can be in the header or footer, but would be okay
on a contact page that is clearly marked with a contact or similar
- Optionally use HTTPS and obtain a certificate from a high quality
certificate provider. A cheap provider often is not trusted as
- Make sure you are using a quality registrar. They are not all created
- Make sure you are using a quality host.
- Make sure that your domain name and IP address are not blacklisted.
This list is just for starters. I created a full list of trust elements found in this answer: How to get Google PageRank (from the toolbar) to increase from 0?. There may be more ideas there.
Site rank will come naturally with any 301 redirect and trust score.
Page rank will come using a 310 redirect from the old domain to the new one. For example:
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]
This is a decision that only you can make. But hopefully, I gave you some ideas.