There are many trainers out there. All will work and all provide more than enough resistance for just about everyone. Sprinting might be an exception.
Resistance units vary considerably. They are:
1) Fan, or "turbo." A small fan or dual fan catches air and increases resistance exponentially, the same way the wind slows you on the road. Strengths: Exellent road feel, good resistance, inexpensive. O'Bree used a cheap turbo trainer that was several years old to do the majority of his hour-record training!!! Weaknesses: LOUD. What??? Fans are LOUD!!!
2) Magnetic: The "mag" trainers. Some have very high resistance (Tacx). Magnetic fields create the resistance, and moving them closer together creates more resistance. Strengths: Quiet, strong resistance, fairly cheap. Weaknesses: No exponential increase in resistance. Inaccurate road feel, mags are not very smooth.
3) Fluid: Fluids are newer than mags or fans. Fluid sealed in a chamber creates resistance against impellers inside the unit. Strengths: Great resistance and road feel, very quiet. Weaknesses: LEAKS!!! Cheap fluid trainers will leak almost guaranteed, spilling oil on your floor. The best is the Kurt Kinetic. Expensive, but worth the money.
4) Friction: There is a new entry, the 1UP trainer. Many claim it is fantastic, with excellent feel, quiet, and great resistance.
5) Electronic: The Computrainer, Tacx, and Cateye are most common. Strengths: Quiet, wattage data, good resistance. Weaknesses: Expensive, possible accuracy/calibration errors, same road feel as mag units (non-exponential).
Any of these will work. By far the most important variable in your fitness is how hard you train. Doing hard workouts on trainers is one of the absolute best ways to get fast on your bike. Whatever trainer you get, make sure that you will use it!!!
Also, it is very important to use very powerful fan(s) to cool you. You will generate tremendous body heat (unhealthy) and puddles of sweat without one.
The most important aspect of trainer work is knowing exactly what you are going to do and for how long. For example, a 20 minute warmup, progressively raising pedal cadence, then 6 x 4 -5 minute intervals at 92-95% of max heart rate, then 20 minutes of cooldown. Having a plan breaks up the workout so you only have to focus on a few minutes of a given activity at a time. Don't just jump on the trainer and ride for an hour. You will burn out mentally very quickly that way.
You can spend $100 on a fan unit to well over $1000 for a high-end electronic unit. Figure out what features are most important. $100 fan units work great but again, are very LOUD!!! With all the other trainers, you pretty much get what you pay for.
Racers absolutely need a trainer to warm up at races. I recomend a very cheap, used fan or mag unit for this. The more beat up and worn out (as long as it works), the better!!! Keep your high-dollar trainer at home. Thieves love to steal trick equipment, especially when you are in the middle of a 60 minute crit!!!
Don't be afraid to spend $300 or more on a good trainer, especially if you are serious and will be doing lots of hard workouts!!! With trainers, you really get what you pay for!!!