skilled worker 3 or 5 years

Hello,
I've got an offer from an employer for a long-term service. However, the employer asks me whether I will apply for 5-year visa OR 3 years visa plus further extension. Does anybody know pros and cons of both options?
I understand that total costs are more or less same except they are upfront or long-term. But may be there are some issues with visa extension after 3 years?
Any advice is welcome, thank you.

The fees are not the same, actually there is a considerable difference between them Home Office immigration and nationality fees

Also you would need to pay IHS accordingly, it is paid depending on the length of the visa you're applying for.

I agree that difference is considerable in the beginning. But 3 year visa will require extra fees for prolongations which result in similar amounts after 5 years. Am I right?

Since the employer is offering you these options, the lenght of the visa seems to be a personal decision, including your payment capacity.

I wouldn't do the maths in that way. Whatever the costs are now, it definitely won't be the same in three or five years time. Also currently there's an inflationary environment and I can tell you it doesn't work that way.