The way I see it then yes, it's just as bad if you decide not to save someone *if* you're able to (no one will expect you to save a drowning man if you don't know how to swim).
Now, whether it's just as bad, I'd say it depends. Taking the example above, you'd judge differently if the person not helping was a passerby than you would if he was a lifeguard, right? Certain people have the duty to help the rest (doctors, policemen etc etc) so they're judged harsher.
It also depends on whether you were responsible (and how much) in endangering the one you didn't help. See: Not helping a biker that's hanging with one hand from a branch above the cliff Vs The above + you were the one that drove on him and knocked him.
On a relevant note, I found a law saying that whoever willingly avoids helping someone facing mortal certain even though s/he can without danger of his/her (read: the one not helping) is punished with one year in jail. Meaning that not helping is not only frown upon but also punishable, even under these specific circumstances.
Totally not related:
Did I ever say that I just l-o-v-e the Ayanami Rei pic below the rules in the right of the typing box when you're replying? I always had a soft spot for her and that one is so funny...