This depends on what kind of riding you do. If you do mostly light trail riding, fields, and stuff like that, you can probably get away with just offset wheels or wheel spacers. If you're a more aggressive rider, ride in a lot of rougher trails, do a lot of jumps, etc., it would be worthwhile to get some extended a-arms and aftermarket shocks. Of course both options have their strengths and weaknesses.
With either one, the extra leverage created by the extra width will cause your shocks to be weaker, which can mean both bottoming out easier, and getting more roll in the corners. Setting the preload stiffer can lessen this, but especially with extended a-arms, it's best to go with a good aftermarket shock. You'll thank yourself for making this purchase, it's well worth it. A good set of aftermarket shocks will increase handling and jumping abilities, and will also smooth out your ride quality, which also allows you to ride quicker because you have more control and don't get tired so quick.
The big benefit of offset wheels or wheel spacers is price in comparison to a full front end setup. There are quite a few downsides though. One of the big ones is that it does stress a lot of the front end components, including wheel bearings, ball joints, and that extra leverage makes it a lot easier to bust a tie rod end, which i can tell you from experience really sucks when you're out in the middle of the woods. You also get an increase in bump steer (the tendency for rocks, roots, etc. to pull your wheels in the direction they want you to, which causes you to fight your handlebars more and wears you down quick.
The biggest benefit of extended a-arms in comparison to cheaper methods is it doesn't create all the unwanted side effects like bump steer and excessive stress on components. You also get a small increase in suspension travel, some caster/camber adjustability, and usually a bigger increase in width as well. Although this is obvious, they also look better than stock ones. Most aftermarket a-arms also have lifetime warranties against damage, so if you would manage to bend or break one you can get it replaced for cheap or free. Another benefit is that aftermarket a-arms, unlike stock ones, have replaceable ball joints so you don't have to replace the entire a-arm if one breaks or wears out.
Hope that helps you make your decision.