First of all, technically, integer is preferred (1, 2 or 3 for instance). Secondly, Sybil issue is not relevant because there is no difference between one big vote from an account or aggregated small votes from multiple accounts.

The only issue with linear system is self-voting. Regarding this, please see my comment above. IMO, self-voting is not a serious issue in the linear system. As @sigmajin well pointed out, the basic assumption of n^2 is equally distributed stakes, however this is not the case in the current Steem.