Is DVI better than D-SUB

If I interpret what you're saying correctly, the "D-SUB" is the 15-pin VGA connector. If so, then yes, the DVI connection would be better than the VGA connector. The reason is that DVI is a completely digital signal and the VGA is an analog signal. Analog signals are susceptible to things like ghosting and inaccurate colors. However, with a high quality VGA cable, particularly if it's kept short (under 6') you probably will not see the difference on a small monitor. Your Viewsonic, being a 24" model, the possibility of ghosting or blurry letters at small font sizes increases. But I have seen very high quality done on 22" VGA monitors so it is possible.

If you have the DVI cable already, then use that, assuming you have the appropriate DVI adapter for your Macbook. But if the VGA image looks good already (i.e.: letters are readable and not blurry) then you might not see any difference switching the cables. I suspect that you would see a difference so additional cost of the DVI adapter for the Macbook would be worth the gamble IMHO.

I think Amazon sells the connector for as lowest $5.85 and apple from $19.49, does this make a difference in image quality? Should I go with the more expensive one?

They shouldn't make a difference, but in one case a client of mine had a non-Apple adapter and it didn't seem to fit the Macbook so well. It worked, but was loose and seemed to pop out too easily, so you had to be careful not to hit the wire. Me personally, my adapters are Apple ones, but that's mostly because I have an Apple store nearby and shipping to Hawaii would offset any savings. So the "extra guarantee" of Apple quality is some comfort for me.

If the image looks good with the VGA which is my case (it looks good),I wont be able to see any difference?

If it looks good, then you might not see any difference. But then again you might not know how good it could be. Imagine it like the first time you look at a HDTV. Standard def TV has worked well for over 50 years so why do we need HDTV? But everyone gasps when the see HDTV for the first time, commenting on how they can see every grass leaf, every strand of hair, every pimple on the actor's face. 😉 So we never knew what we were missing. Same with a large LCD screen (large being anything over 19".) In your case the difference may be subtle, but when you're trying to view 5 point fonts, DVI it could be the deal maker. Like I said, IMHO, even if you gamble and pay for the Apple adapter, I suspect you'd see a difference. The improvement might even be just psychological, but to me that is worth $20. 🙂

Awww, you're making me blush. 😊 But you're welcome anyway.