Dota's wards are pretty different from LoL. The reason League of Legends used to devolve into a vision battle was that both teams could just keep placing wards as long as they had gold. So one team's support player would just go around placing wards and removing any wards they found, while the other team's support player would go around re-placing their destroyed wards and destroying the other team's wards that went up in the same place. Basically, it was a giant vision circle. Dota never offered unlimited vision.

Here's a quick rundown of the three systems:

Dota:

Observer Wards: Invisible, last seven minutes and give vision of a fairly large area, but there are only four available to each team at a given time. Cost 150 gold for a stack of 2.

Sentry Wards: Invisible, last four minutes and reveal invisible things in a fairly large area, but don’t grant vision of an area (so you need to find some other way to get vision of the area where the invisible thing you want to see is). Cost 200 gold for a stack of 2. There is no limit to Sentry Wards.

Old League of Legends:

Sight Wards: Invisible, last three minutes, give vision of a fairly large area, take three hits to destroy. Cost 75 gold. Unlimited.

Vision Wards: Invisible, last three minutes, take three hits to destroy. Give vision of the same area as Sight Wards, and also reveal invisible things in that area. Cost 150 gold. Unlimited.

New League of Legends:

Sight Wards: Invisible, last 3 minutes, give vision of a fairly large area, take 3 hits to destroy. Each player can only place three Sight Wards at a time. Placing a fourth will destroy your oldest ward. Cost 75 gold.

Vision Wards: Visible, last forever until destroyed, give vision of the same area as Sight Wards, reveal invisible things, take 5 hits to destroy. You can only place one Vision Ward. Cost 150 gold.