And to provide an opinion on which would be best to visit you have to comment on the differences between the two, even though tiger temple lovers will criticise you for it. Everyones entitled to an opinion even if you don't agree with it.
Stick with the cycling tour. You'll probably get to see parts of Thailand that are off the usual tourist trail. The floating markets are good fun and by visiting farms you'll get an insight how rual people live. Love the idea of riding the ox cart through the paddy fields. Plus you'll get a chance to burn off all the calories you'll pack in sampling lots and lots of yummy thai food. We drove through Vietnam and Laos a few years back stopping off here, there and everywhere and I found it a much more enjoyable experience than flying from city to city and hopping around the usual tourist sites. You get to see a hell of a lot more and have a lot more interaction with the local people. I'm guessing the cycling tour would provide a similar experience.
All you'll get at the tiger temple is a long journey, be corralled into a queue with other tourists in 40 degree heat just to hand over big bucks just to have a quick photo with a tiger or if you're willing to stump up more money, the chance to smother some poor cub with shampoo. The cycle tour sounds fascinating whereas the other is money making trophy photo tourism at its worst. However, the river Kwai part of this tour would be interesting particularly if (like me) you know someone whose father was captured and forced to work on the railway and lost some good friends along the way.
In terms of booking now or later, we've always booked in advance as the tour costs are much of a muchness. We've always used SalaThaiTravel and never had to pay deposit up front. We just pay in cash on the day to the tour guide.