I'm assuming you are talking about the 1979 through 1982 Blizzards. The 5500 Blizzard had the model 503 fan cooled, single-piped motor. The 7500 Blizzards had a liquid 340, twin piped, and the 9500 had the liquid 440, twin piped. Ski Doo did continue with that design with a larger 521 liquid twin piped motor in 1983 and 84, called the 9700, but they are pretty rare.
I have owned a 79 Invader for decades, and just got an 82 Blizzard 9500 this past fall. I haven't had much riding time on it yet.
The rear suspension is much better on the Invader. The Blizzard's isn't much different than my 1975 Sno Jet.
The Blizzard sits higher, but that's partially offset by its wider 34" ski stance, versus 31" on the Invader.
The Blizzard is fairly easy to work on, lots of room under the hood.
The Blizzards are probably more rare than Invaders, particularly ones in good shape. They all had a steel belly pan that was prone to rust, and few survivors today have a really good belly pan.
The Blizzards (liquid cooled) are heavier....440 lb. dry weight vs. Invaders roughly 400 lbs.
The liquid cooled 7500's and 9500's that are still in nice condition are worth quite a lot today. Probably in the 1000 to 1500 dollar range. This is probably due to Ski Doo still being in business, and I doubt that they manufactured as many Blizzards back then as Invaders. I see lots of Invaders for sale on Craigslist for anywhere from 300 to 800 bucks.
Those liquid cooled motors are smoking hot in the Blizzards. The 440 puts out around 80 horsepower, a fair bit more than the more mildly tuned Invader at 68-70 HP.
Parts availability probably about equal between the 2 brands.
The good, old, square shaft drive clutch on the Blizzards was prone to wear out the center bushing rapidly...many times within 1000 miles, but parts are still available, or you can always change to a Comet clutch.
Lake racing...pick the Blizzard...amazing motor.
Chance that the value will increase over the future years.....Blizzard
All around trail riding....Invader....better ride and handling, less "peaky" powerband that is more forgiving.
Both good sleds...the liquid cooled Blizzards were made to do one thing, and they did it well....beat everybody else across the lake.