I have three boys potty trained (one 6mo in the wings) and two daughters (3 year old in training).
They all progressed very differantly. They alos all showed differant signs of being ready. My oldest son was 3 when he began using the potty and to be honest it was a easy miracle type training session... Basically I bought him underware for his third birthday(I had been trying pull ups without luck for about 6mo) told him he is a big boy now and here are your new big boys pants I told him that spiderman wanted to stay dry... he never wet his pants again. From that moment he was dry both day and night. All of my other children were very differant and not so easy.
I found that I was the one who was trained more then they were sometimes. I would take them to the potty every hour or half hour getting them to go on the potty... so yes they would go on the potty, but they really were not ready it was just by chance. by the time I had my third child to potty train I didn't want to be running to the potty with him every hour... like my second son... I read up on the whole thing.
I found out that they do have very specific signals to show they are ready... They have to have a dry diaper for at least two hours at a time. They should be able to pull up and down their pants, they need to be able to have the communication abitily to say they need to go and the understanding of what pee or poo for instance is and when they need to do either of them. Also, not force them or make the potty seem like a punishment. If potty time seems like time out time it will make the transition very difficult and you may get resistance or fear from the child. Be supportive and encouraging but not forceful and frustrated is my best advise. As with everything when it comes to kids I have found that the more you get upset the more they refuse..LOL!
I have had both the big potty and the potty chair for my kids... the big potty once they figured out that they wouldn't fall in was usually their more preferred potty option. The seats with characters was also fun for them it made it a special thing they got to pick out their own potty and it seemed to make it a fun grown up type thing.
Also, one other bit of advise I didn't even think about with my boys... make sure that they go to a mens bathroom from time to time to also understand the Big Man's toliet..LOL! My sone didn't ever go to the men's room he always went with me to the bathroom... when he went to a men's room he was very confused about the potty's until he saw the toilet... also since they have urinals at school... he was happy to not be the boy that they made fun of because he didn't know how to stand and pee or use the big mans toilet...LOL!