I ended up staying at Sobieski for my Warsaw trip. However, it turned out that I would spent one more night in Warsaw few days later on my way from Minsk, so I booked Centrum for this short stay to compare them.
Location-wise, both have a good transport links and I'd say they are on par in regards of getting to places. Centrum's neighborhood is nicer and I quite enjoyed walking to the hotel. Sobieski has much better transportation options to the airport, which I appreciated when I carried large suitcase.
While both hotels are old, Sobieski is more or less completely renovated, although design looks bit like if it was unfinished. Centrum feels old and room has a unusual layout - I still haven't decided if I like it or not. Bed is put in a diagonal position and is facing a window. The view from bed is nice, but on the other hand, this solution turns quite spacious room into a room that has many small spaces instead of having at least one large space. Also, there is very little natural light at the desk due to the room layout.
Centrum has a very comfortable bed. Compared to extremely soft beds in most Carlson properties (including Sobieski), bed at Centrum is harder and provides much better sleep. I also liked pretty deep bathtub at Centrum.
Breakfast was definitely better at Sobieski. I also had a dinner at Sobieski, which was very good (no experience at Centrum).
Staff at Sobieski was hit-n-miss, front desk was very good but most of staff in restaurants was cold. At Centrum, it was quite bad. No friendliness at all and breakfast staff can only be described as grumpy - it just increased the general retro feeling of the place. Interestingly enough, the only person at Centrum that actually smiled was a receptionist with a "trainee" badge who checked me out.

I requested room with bathtub in both hotels and got it at Centrum. At Sobieski I got room without it, which was fine (I understand that such room requests are subject to availability). What was weird is that when I inquired about it, they told they don't see my request, although when I opened the reservation on CC website, it was there. They couldn't reassign me to a room with a bathtub since they were fully booked but they offered to move me on the next day (and were actually very proactive about it, they even left a message for me at the breakfast). I declined since I didn't feel like moving my stuff but it left an impression of them caring to fix it.

Finally, I became Gold during my stay at Sobieski so Centrum was my first stay with the new status. I had a small box of chocolate pralines in my room and printed note with my name added by handwriting (compared to completely handwritten notes I got as silver at some other hotels). No upgrade though - it was actually pretty bizarre: On check-in I was asked whether I wish to add breakfast for 90 PLN (which I did) and then offered an upgrade to Business Class room for 120 PLN as an alternative (basically offering me to get a BC room for extra 30 PLN, which is approximately 7 euros). I declined - 7 euros is a small amount of money but since this was a very short stay and I didn't need really anything other than bed and a bathtub, I didn't see any reason to buy it. Even though I couldn't care less about that upgrade, I found the experience a bit cheap. I mean, if you have a Gold, who already agreed to pay 3/4 of an upgrade price (90 of 120 PLN), why not just upgrade him to earn his loyalty (especially with another CC property in the city) rather than trying to squeeze extra 7 euros out of him? (Bottom line is: I came to Centrum from Minsk where I stayed with DoubleTree. It was my first stay since joining Hilton Honors and yet receptionist thanked me for my loyalty during check-in - something that Carlson Rezidor hotel has never done in my life. It doesn't cost anything and makes a big difference.)

Overall, I found Sobieski to be a better experience and it's usually (though not always) cheaper, so I'm more likely to stay there in a future, But I'm not completely ruling out Centrum, there were good things about it.. However, both stays were somehow below my expectations from Blu, so following my wonderful experience from Minsk, I will also look into DoubleTree Warsaw, which often has rates similar to Centrum and great reviews.