To what purpose to you want a cadence counter?

Are you intending to race and want to work towards increased cadence in certain gears and therefore increased speed?

Is it because you are interested in how fast you can make your legs go?

Is it cause "everyone" seems to have one so maybe you should too?

Is it cause you think its a cool idea or cause you like to record stats?

'puters with cadence counters tend to be a tad more expensive, and are more difficult to mount on a road bike as the need to be rear-mounted.

I use a cadence counter on my training bike only to try and improve my spinning - but I only measure that on certain rides. People can get quite obsessed with cadence counting and sometimes it is better just to ride to what your body can do - ride to "perceived effort".

I wouldn't bother with one on an indoor trainer either - unless you had a specific purpose in mind wth your cadence workouts.

So, I suggest you identify why you need one, and make your decision based on that.