I have used both. It is not so much that they help in getting an erection you still must get stimulated, but they help keep the blood in and you hard. I use only half of the dose I buy and it always does the job. The erection has always gone down slowly after, but can also come back easily. Remember your teenage years when you could get stiff just looking at hot girls. The warnings are for people who have adverse reactions to the pill which is why you should talk to the doctor. For me the side reactions are a stuffy nose and a mild headache. I just keep the dose low which is why I cut my pills (low dose pills are just as expensive as full dose so I buy full and cut in half). If your doctor says its ok to use with your meds then I wouldn't worry to much the person more likely to get into the problem you see warning about is someone who does not need to use the pill but uses it.

It is possible with your new meds that side effects will diminish over time.  That'd be nice.
For me, I can get wood very easily with viagra but after orgasm it goes right down. Just like any drug, it affects different people in different ways.  As with all disclosed negative side effects, they are generally uncommon.  I believe the 6 hour deal is rare.  And I think it is as much an advertising tool as warning.  "Man!  I'd LOVE to have a 5 hour boner!", etc.  Follow doctor's orders and you'll be fine.