A comparison of cryptographic keycards

Unless smart card hardware has improved drastically in recent years, you can't make them fully tamper resistant against physical attacks (power analysis, timing analysis, fault injection, etc.) without software designed to do so, and open sourcing the software makes the attacks considerably easier. Effectively, security by obscurity adds a required extra layer of protection against physical attacks. Either you use closed source and trust the software vendor (and possibly e.g. Common Criteria evaluation), or you use open source and trust you'll never misplace the physical device. It boils down to your security requirements, and which approach provides more assurance that your requirements are met.

Disclaimer: I used to develop smart card operating systems for a living, but it's been a while.