My 2 cents:
Dont spend too much time devising methods against piracy. Use simple serial generator mechanism to unlock the game which user can enter manually during the execution.
I would say have a single build which can switch to trial mode or full based on the serial key entered. This will reduce the overhead of maintaining two separate code.
It is a personal opinion, that people who really buy software at first place will buy your game no matter how many pirated versions are available. So make registeration purpose as simple as possible which will deter a normal user from cracking it and at the same time easy to use. Hackers will crack it no matter what protection you use it. Otherwise we wont see the pirated copies of microsoft/adobe products etc who spend so much on making their products piracy free. No matter what they charge, people do buy it. Its the quality of the product which will encourage your users to buy your product.
Also, try not to impose locks on your software like using MAC address etc for generating the serial numbers etc. Online activation may be a good idea but remember people are skeptical as to what information you try to transmit while activation. Also you might have to provide an alternate mechanism for offline activation if your customers dont have internet connection or work in separate LAN.
Once you see that you game is getting popular and you see more pirated copies with users, you may invest more time and money on developing anti-piracy techniques.