Kuddos for researchign before buying an impulse betta!
Since you're getting yourself informed first, you'll avoing buying unnecesary or innadecuate things.
Here's my two cents:
First of all YOU NEED A HEATER, no matter what tank size you end up with. It's a must.
You're betta will be just that, "okay" in a 1.5 gal. In that tank, you don't need a filter and you need to so 2-3 weekly water changes and I mean 100%. BUT this depends on how much and what type of food you feed your betta, if you remove uneaten food, etc.
It's useless to have a filter in this size tank because you wont be able to maintain a healthy bacterial colony that will keep your tank cycled. Therefore, the substrate you use doesn't really matter.
Gravel is obviously harder to clean, specially if you're going with the multiple water changes a week, plus is goes down your pipes if you're not careful.
Why not buy a bigger tank? Walmart has 2 very reasonable 5 gal tanks for $30. These come with pretty much everything you need but the heater and decorations. Check it out online!
You can cycle tanks that size, which will reduce your number of water changes by A LOT and they only occupy 1-1.5 square feet of space.
If you're cycling then you worry about what substrate to use and you'll keep your fish very happy with a more stable water quality.
Go to the Betta Fish Care section, there's lots of info there!
Check out this post: http://www.bettafish.com/showthread....e=0#post441600