Conditional assignment statement vs selected assignment statement

In this instance there is no difference, the final logic should be the same.

But with select will always be a normal mux, as all conditions have to be covered.

The second example can be used to infer all sorts of logic. In this case it's a priority mux, as r(3) has priority over 2, etc.

but you could also infer a latch:

code <= ip when en = '1';

or some other logic:

code <= a when ip1 = '1' else b when ip2 = '1' else c;