Compound movements will always give the best possible results in regards to achieving muscle mass and strength. Give the choice of doing compound exercises like squats, dips, Dl's, etc for three months or doing only isolation/leverage (isolation...misleading term) exercises like flys, crossovers, straight arm pullovers, leg extensions, laterial raises, etc, whom after that three month period would have gained the more pure muscle mass and strength? PL'ers come to mind in this regard. If biceps curls, using BB's or DB's, is an example than these are not true isolation movements. Both will have the influence of the anterior delts, in fact the whole shoulder girdle as well as the forearms & back area.. Even the Scott curl and the concentration curl would not be considered a true isolated function.
One of the better bicep mass builder would be close curl grip (4-6") chin, touching the upper chest each rep to the bar. A second choice would be the BB cheat curl, a very effected muscle builder. The Pec deck, when the path of the resistence is slightly above the shoulder level (anything lower does not have the same affect on the pec's themselves) can be very rewarding . Adjusting the seat height may make up for any incorrect angle to this exercise. The stress of the workload/resistence is on the point of the inner elbow, about shoulder high. . Some machines require the trainee to grip handles out and away from the body, which in this case the exercise can be a less affective as regular DB flys or cable work.
As I said in the above post, "Declines pretty much mimic regular dips". Did not mention that it was exactly the same as dips. In my view declines will always take a back seat to dips anytime. Good Luck.