The 830 has been discontinued; increasingly difficult to find. The 840 Pro is newer, about the fastest consumer SSD out there (in reputable benchmarks) with a longer warranty (5 vs. 3 years) than the M4. But there is likely a higher price (depending on the vendor) accounting for this.
More comparable to the lower-line M4 than the higher-line 840 Pro is the 840 (non-Pro), which is the replacement for the 830. Lower price with lower performance (better than the M4 in the Anandtech benchmarks I've seen, though) & shorter warranty than the Pro.
Thanks. The information is very helpful. It's quite likely I might get the Crucial then.
What I don't know is what is the warranty policy of the Brazilian Samsung branch when SSD is not bought in Brazil. Or Crucial if there is a rep. Brazil is where I will bring these drives to.
Seagate and WD do not care where you bought regular HDDs, but warranties are shorter then in the USA. Usually 2 years. I wonder what's Samsung policy.
Can't help out on other-country warranty issues, but its best to contact the manufacturers about it. I do know that some warranties cannot be utilized when you are in a country other than where the item was purchased, but some definitely can. As well, as you say, the manufacturer having an office there.
For recommendations, check out benchmarks. The 840 Pro (substantially) & 840 (less so) are both faster than the M4 (they are newer, use newer NAND tech, different controllers, etc.). I'd say the 840 non-Pro is the best choice (if you can transfer the warranty) for the money, unless you want to pay extra for the performance (and potentially longer local warranty) of the 840 Pro. I'd put the M4 in third place.
As to Canadian availability, the all of them are there. Check some meta-shopping sites like Shopbot.ca & Pricebat.ca.