Firstly - the similarities.

1. They are both based mainly on off-road vehicles. However, while 4x4 Evolution (understandably) features only 4x4s, Insane has a much wider range of vehicles - from Beach Buggies to 12 wheeler trucks! I myself believe that variety is the spice of life, and believe that the wider range of vehicles is a good thing.
2. They both allow you to go 'off the beaten track'. The difference between the two being that 4x4 Evolution only lets you do so, while Insane encourages you to, with checkpoints placed at random throughout the levels on certain racetracks - you could be racing up a steep hill for the first checkpoint, then having to take a U-turn and racing back down the hill to get to the second. Thanks to this, Insane proves to be a much more exhilerating ride than its competitor.

Now - let's compare other aspects of the games.

1 - Multiplay. In this, both games have their advantages and disadvantages. 4x4 Evolution first - it is one of the only games to allow Dreamcast owners go up against PC owners and Mac owners, and it runs quite well on even a 56k modem. However, it features one unforgivable problem - the cars don't make contact in multiplay - you can drive straight through them! You may as well be racing against ghosts.
Now for Insane - while it doesn't have support to go against anyone other than other PC users, it has two things to its advantage. 1 - it zips along on a 56K modem - great for people in the UK who haven't got cable/broadband yet. 2 - The cars can collide.
On this evidence, I declare Insane to be the better multiplayer game.

2 - Single Player. In both games, this is the main problem. Being racing games, they are best played against humans, due mainly to the fact that computer controlled cars generally lack the unpredictability or skill of a human opponent, though Insane does attempt to address this fact by having a good A.I which sees the computer-controlled cars not only finding their own routes to certain areas depending on what car they are in, but also sees them making mistakes, thus leading to a much less artificial feel than the majority of racing games. Also, while 4x4 Evolution only enables you to gain extra cars/tracks by playing online, in Insane you can do so offline, meaning there is actually a real reason to play offline.
A well-deserved point to Insane!

3 - Graphics. Insane rules the roost on this aspect - the cars are not only good looking, but they are well animated too. It's wonderful to watch them bouncing around the landscape, with the wheels acting as though they have a life of their own, bouncing all over the place. 4x4 Evolution is... functional. So thats another point to Insane.

It's at this point that I realise how much space this takes, and that I realise that now would be a good time to stop, so I'll round off my review now. Basically, Insane is by far the better game, and has many other things going for it which I haven't had time to mention, such as the real-time damage to the cars, the wildlife, the physics... the list goes on. Quite simply - BUY IT! Two thumbs up!