Don't worry about past mistakes too much. I did the exact same thing when I got my first betta. All my info came from friends who were making the same common mistake. But now your next fish can be happier 
I, personally, would choose the 5 gallon hex out of the three you named. All three would be fine for one betta.
Soft, fake plants are just fine. Also, speaking of decor, you might want to add a cave or two for your fish. Bettas enjoy them.
You really can't cycle a 3/5 gallon tank so don't worry about that.
I don't really know anything about that heater but I'm sure someone here can help. Maybe you could read some of the reviews here: http://www.amazon.com/Elite-Submersible-Preset-Heater-25-Watt/dp/B00106X8QG
It is 4 star rated with 60 customer reviews.
Good luck!
I, personally, would choose the 5 gallon hex out of the three you named. All three would be fine for one betta.
Soft, fake plants are just fine. Also, speaking of decor, you might want to add a cave or two for your fish. Bettas enjoy them.
You really can't cycle a 3/5 gallon tank so don't worry about that.
I don't really know anything about that heater but I'm sure someone here can help. Maybe you could read some of the reviews here: http://www.amazon.com/Elite-Submersible-Preset-Heater-25-Watt/dp/B00106X8QG
It is 4 star rated with 60 customer reviews.
Good luck!