That model only has a 1Hz update rate so it will not provide particularly accurate lap timing. You could only expect your lap times to be accurate to within about 0.25 second I would say.
The qstarz bt-q818x (5hz) seems to be the model that most RC users go for and it works really well so that would be my recomendation. About USD$70.
Logic says the q818xt would be the better option but since its a fairly new model there hasn't been much feedback on this forum yet that I have seen that suggests you will achieve improved laptiming performance.
They are only about $15 extra, so if you are not too stingy then you certainly wont be doing any harm by purchasing the faster receiver.
Think how far you travel in a second on your MX bike. At 30mph (48kph), you'll travel over 13 metres every second. What's the top speed in an MX event and how far apart do you want your samples? For me, I'd only get a sample every 50 metres with a 1Hz receiver. For lap times, you may be fine, but for any useful information, that's not a high enough resolution, in my opinion.
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The qstarz bt-q818x (5hz) seems to be the model that most RC users go for and it works really well so that would be my recomendation. About USD$70.
They are only about $15 extra, so if you are not too stingy then you certainly wont be doing any harm by purchasing the faster receiver.
thanks.