You always choose what is the best.
Good thing is you have not started, so you saved there IT group's time, they do not need setup your PC.
I will send another mail say: I have to reject the offer.
It is very easy.
There is nothing to worry about.
In the future, if that Agent can sell you , he/she will.
If that company need you again, they will hire you.
Again, you choose what is the best 4 U!
Thanks!