But one thing is for sure, NEM has something completely different than NXT's PoS, it is called PoI and is quite a bit more complicated. I have tested it on the Beta so I can see how my chance at making a block goes up and down and not just accordingly to how much NEM I have at the moment.
Actually, this is a disadvantage - a novel Proof-of-X that wasn't tested in the wild nor publicly peer-reviewed. People think that they can add anything to "Proof-of-" and get a new way of securing a blockchain.
A scarce resource must be put into the base of any Proof-of-Resource because it's the only (known) way to counteract Sybil attacks in a decentralized system. In Proof-of-Work it's hardware that do hash computations. In Proof-of-Stake it's coins (effective balance). This is the first important point.
The second important point is "insularity
*" of the system. Bitcoin is not isolated, at any moment someone could come out of the blue with pockets full of ASICs and do a 51% attack. Nxt is isolated, new coins can't be created, this protect Nxt against 51% attacks of the "outer" world.
Note that Proof-of-Stake principles may vary. Peercoin's PoS uses coin-age, this allowed (not sure if it's still actual) to wait several months to collect enough "power" and attempt an attack, this is why time component was capped at some value. In Nxt this flaw was removed by removing time from the equation completely. Time is not a scarce resource, we have plenty of it (let's not "fork" to a philosophical discussion).
There may be other important points but I can't recall them now. Regarding the topic of this thread - Nxt conform to both the points perfectly, only scarce resource is used and it's isolated. If NEM will be successful depends on how "strong" its technology is. Even with perfect marketing it may be killed by someone like Luke-Jr if a vulnerability is found.
...not just accordingly to how much NEM I have...
is a red flag. NEM's forging algorithm should be reviewed as soon as possible. Without such a review "Will NXT or NEM be the most successful?" doesn't make much sense because we can only guess if NEM is capable to survive the first months of its existence in the wild. We already have an example of a coin that used a novel Proof-of-X algorithm and now is traded below the initial price because of problems related to flaws of that algorithm.
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* - this may be an incorrect word, I wanted to use a word that shows if a system is isolated
I am still going to stand by what I said in that PoI is more complicated that NXT's PoS. I might not have been clear, but I was trying to say the writing for the code is more complicated. As BCNext explained well, NXT's PoS means that each coin is like a little miner. Now.... I don't know anything about code, but it seems like it would be much easier to code that then something like PoI which has multiple variables. When I wrote the post originally I was just trying to speak to the fact that NEM isn't a clone and was trying to give examples of how the code could be different.
Now that said, I agree with much of what you said. As of now the NEM test net has been going for a couple of months, so I think they devs are hopefully thinking about some of the things you mentioned. If a weakness is found that is devastating.
And yes NEM needs to have its code opened up and reviewed, not just the PoI, but all of it. All coins need this, especially ones with novel proof-of-x. I would personally like it if you reviewed NEM's code. I know that the devs of NEM were looking at the code of NXT when it first came out, I think it would be really cool if you looked at the code of NEM when it first comes out. I am completely sure you can have valuable input being you have been thinking about this kind of thing loooooong before NXT came out. And to that point, I can't really argue with your points above about what makes a proof-of-x strong or weak. I am sure you know way more about it than I do.
Also, I think the whole "Will NXT or NEM be more successful?" debate is far too premature. NEM hasn't even had its Genesis yet. Lets get a live blockchain and an open code and then come back to the discussion. NEM needs to have its genesis and open its code to be respectable. I think the devs know this and I have heard them mention things to the extent so I really hope it happens.