Okies time for me to donate my 2 cents to this vid =D
Marth - I noticed that you didn't use alot of grabs. Its been said before but Marth + Grabs = pwnage. Also like shadow also said, more aerials would be nice. Marth in my opinion has some of the best aerials in the game, because you can use them to juggle and rack up massive damage as well as knock a dent into your oponents stock by doing the all mighty, Ken combo.
When recovering, you should try doing only the first hit of the Sword Dance. This alows you to switch to the UP-B a little easier. Speaking of the sword dance. It's deadly. If done well enough it can win you the match, but it can also put you in the worst of spots, so be careful. Personally, I use the sword dance alot, BUT, its something that takes alot of practice and you gotta be oportunistic with it.
In my opinion Link is possibly the most adaptable character. That however is a double edged sword. My rival and good friend, plays one mean Link and my Marth can barely keep up. You just gotta got at him from odd angles. Remember to block the bomerang with your sword whenever possible and dodge the arrows as much as possible. Adaptability leads to success.
Link - You seem to have the bommerang down pat. I am a noobie Link player but I can tell you that playing a good Link wether it be Young Link or old, mind games are the KEY to success. Learn some advance techs and start to work on your aim a bit more. You should however smash a little less. Links smashes are slow and open up oportunities (specially against characters like Marth, Fox and Shiek) for some really powerful combos that will simply dominate. If you know you can't match the person in up close combat, switch over to your long range game, might take a bit longer, but if its good enough, you should be able to own your oponents.
To Both - My best advice. Get your advance techs down (Atleast wavedash and L-cancel) and hit your local tournament place or smashfest. I've been playing smash for under a year and I can hang with the big boys, so go out there and show them who's boss. Good luck to both of you!
