Is it better approach to declare and initialize a bigdecimal number with "new" keyword or static method?

Well clearly createBigInteger is doing more work - it's checking for nullity, when you know the argument value won't be null anyway. That's only a tiny, tiny bit of extra work though - almost certain to be irrelevant in reality.

I'd be surprised if this were really a performance concern anyway though - have you identified this to be a bottleneck in your code? If not, write the most readable code - which for me would be the constructor call. Then identify what your performance requirements are, and test your whole system against them. If it's not performing well enough, write more tests or use a profiler to identify which areas are causing problems.

Another alternative would be to use the BigDecimal(int) constructor - why bother parsing a string?

BigDecimal num = new BigDecimal(123);

If you wanted, you could even have this as a constant, so you could reuse the object:

private static final BigDecimal DEFAULT_FOOBAR_VALUE = new BigDecimal(123);

// In a method or whatever...
BigDecimal num = DEFAULT_FOOBAR_VALUE;

Aside from performance, I'd argue this is clearer as it indicates the reason for the constant.