I think the park rate vote system is the best of the three mechanisms. But what they all lack is some feedback mechanism to the user. When you vote for a motion you want to have confirmed that you vote is taken by the network. Preferably immediately or with a timer waiting for a block confirmation. The block confirmation can be a counter so you will have visual feedback on the minting and how many votes/block you cast during the process. Nice to have would be feedback on the actual vote count, but I understand that will be an external site.
Same for a custodian vote. The custiodan vote also has a not very intuitive way of entering information for the address. The default space for address is very narrow and for amount very wide (would turn that around). It would also be good to provide some idea of what to enter in address in greyish default text, e.g. Enter published motion hash.
Given the relative complexity (new and unknown for many) it would also make sense to provide some in-client help or a link to a website with client help information (wiki or something). This can be added under help in the top menu. Also always wondered why the abouts are under help and that help guides you to the command line which is for most users even harder to get, but that is not Nu specific.
For the park rate votes you can also think of some visual feedback e.g. with colors or indicator whether the interest is rising or lowering (e.g. read previous 10 blocks)
I think the client should focus more on user interaction and ease of use, current state from user perspective nd immediate feedback while an external site would provide status, trends, vote counts, proposals etc. Ideally they will be married up somehow (linking to each other or all in same dashboard) but that is a bit further away I suspect.