Marstrom is 34, Ullmark 30, I would think Ullmark carries more value.
Markstrom at 34 and 35 for ~4 mil a year
vs
Ullmark at 31 (he turns 31 in a month) at 5 mil, then
Ullmark at 32-36 at ~6.5-7 mil
I think Markstrom is a good value contract, ~4 mil is on the lower end for starters, if he can provide average to above average goaltending, that will be a very team friendly deal with limited risk as it expires when you expect a goalie to start their decline.
Ullmark at 5 mil is similarly a good value contract, I think he's more likely to provide good value on the initial year.
After year 1, he's basically giving you market value, top 5 salary (maybe a bit lower once Shesterkin and Saros extend)
The danger comes towards the end of his extension, where you might end up with a top 5-10 salary but not getting top 10 results as he'll be 35-36 years old. Not a lot of goalies are still top notch at 35+.
I tend to agree Ullmark has more value than Markstrom, but he carries significantly more risk, which limits just how much more value he should carry.
From a Sens perspective, you accept the risk associated with those last two maybe 3 years, but you don't pay a price as though that risk doesn't exist.