Which one depends upon the range at which you shoot, your ability with simple math, and your finances.
For distances up to 800 yards or so, a simple wind meter will do. For distances up to 1K, a simple wind meter will do if you can estimate your altitude and the current temperature (hint: water freezes at 32 degrees ) and either enter it into your program or apply correction factors to your drop chart number.
For distances much past 1K, you should get serious about the exactness of everything and buy the 3500.
The Smokin Fur Rifle Club