I'm 190cm tall. As a tall rider I always look for a bike with high stack measurement.
Stack measures the distance straight up from the center of the bottom bracket to the height of the top-center of the head tube. It tells you how high or low you can get the handlebar without making the bike look and/or ride weird (as with a many spacers under the stem, a severe rise stem, a high-rise riser bar, a fork riser puck that whacks out your geo, etc, etc).
I can't get comfortable on a bike when I can't get the handlebar high enough. Maybe not an issue for all tall riders; of us are fine on lower stack frames. Personally I look for stack 645-655mm as my ideal, puts me in a neutral body position about 45 degrees spine without a lot of spacers under the stem.
Many bike companies don't seem to increase stack proportionally as bike size increases. They go longer but not much higher. I can bend my arms to get my body lower if my hands feel high, or on a road bike I can hang out in the drops more. But I can't make a too low bar feel higher by moving my body. I eventually end up sore and unhappy on bike with too low stack height.
Both bikes you list have medium to high stack. 633mm on the Sirrus X 3.0, 648mm on the Giant Roam. The Giant's stack probably is less when you sit on the bike though. The suspension fork will compress a little and the front end of the bike will lower. It's hard to know if a bike company accounts for this in their published geometry specs, but given the Giant's 160mm head tube, I suspect they don't.
As such, the Giant and Sirrus probably actually have similar stack height, Giant probably a smidge higher.
To me, the next most important measure is reach. Reach is like stack, but instead of vertical, it is the horizontal distance from BB center to top tube top-center. Too short and you are cramped and curled over like a jumbo prawn, too long too stretched out with your weight too much on the front wheel. Reach, like stack, can be manipulated a number of way with parts, but also like stack it is best to have the frame's native reach length right for you.
Sirrus is 414mm reach, Roam 427mm. On a flat handlebar bike (not road bar) 414mm is workable for me, but I'd ride a bit of longer stem. at your height the 427mm seem pretty spot on.
The Giant frame is a little bigger, made more precisely for someone your height. This is confirmed by the manufacturer's suggested height range. You are toward the middle of the range for the XL Giant. This is good - brands do vary widely in sizing, but I've found that when I'm in the middle of the suggested range I have a far better chance of feeling great on the bike.
The parts on the Giant range from equal to significantly nicer. The brakes are WAY better and the rotors are bigger. This is quite important if you are big and need to stop more mass. The drivetrains likely are similar user experience function-wise, but you get a much lower low gear on the Giant and much better spread of gears. I love 1x and ride it on all my nice bikes, but I love more not pushing my bike up a steep hill because I can't turn 40-42 gearing like the low gear on the Sirrus. You could ride loaded tours on the Giant as it is geared.
I dislike commuter suspension forks -- generally they are a waste of weight and precision for very little return in comfort over a regular fork. But if it was me, I'd tolerate the suspension fork on the Giant for the superior parts on the Giant frame and the likely betteer fit of the Giant frame. You could always put a rigid fork on the Giant down the road for maybe 100-150 quid - lot of nice quality, tall enough quick release rigid forks on the market from the likes of Surly etc for not to spendy, many floating around second hand parts shops, etc.
Also, since the Giant's suspension fork is an air fork, you can pump it up pretty stiff if you please, and it will act a lot like a rigid fork, not bob around or deflect as much over irregular surfaces.
In short: though the Sirrus is a nice bike that would work pretty well, I'd go with the Giant.
to CC.
