From what I've seen from the compiler, single statement if statements are the same; however, if the if statement block is too large or too complex then the compiler can't/doesn't apply this optimization (and creates separate clauses for each if block) and manual coding is required for better performance.
You can check your particular code by looking at the generator ISA via the profiler or SKA.
So, in short, it's better to try to use the select() function or the conditional operator....this is because the compiler doesn't "pack" across if blocks (else included along with else if, etc, etc...).
If you use the select() or cond. operator for each statement then the compiler isn't going to create extra clauses and you will give the comiler the ability to "pack" across all your statements (instead of along the statements in each block) thereby better utilizing the ALU. Also, you are reducing the amount of clause switching that needs to occur.
Hope that helps.