Hi John,
Either will do the job and yes you can switch. Side effects will be similar. No libido, almost certainly, no erections unless you put in a lot of effort which you can't be bothered to do (you will need some advice on penile health). With bical you may start to develop breasts, this can be prevented if caught early, again others on here can advise. With decapeptyl you may get some irritation around the injection site.
With either you may get hot flushes, and then less common side effects like mood swings, joint pain, and these could be different between the two drugs depending on your body.
I would find remembering to take the tablet daily a bit of a nuisance. I was on zoladex for two years, no major problems. I would just choose one and swap if you get a troublesome side effect .