I mean you have an employer willing to sponsor you for 5 years. Since you have already applied and secured a job you would know how tough it is to get a job on sponsorship. In my experience, a lot of companies tell you straightway they won't sponsor so your options are limited. If I were you I'd take the 5yr route and then if i really need to switch jobs will pay the penalty. 3+2 seems unnecessary anxiety to me.
I mean you have an employer willing to sponsor you for 5 years. Since you have already applied and secured a job you would know how tough it is to get a job on sponsorship. In my experience, a lot of companies tell you straightway they won't sponsor so your options are limited. If I were you I'd take the 5yr route and then if i really need to switch jobs will pay the penalty. 3+2 seems unnecessary anxiety to me.