Endnote XML is a newer and richer format, so may preserve a bit more than RIS when exporting from Endnote. Endnote XML is likely a better "default" format, but either should bring in 90+% of what you'd like.
Note that Endnote has made undocumented changes to both of these and not all people store information in Endnote in the same way. This sometimes prevents getting to 100% out-of-the-box. Inspect records that you've imported. If you notice any undesired behavior, it is better to catch them early. Searching these forums may show threads where people have had similar experiences & there may be a work around (e.g. using a different version of the exporter in Endnote, editing the file slightly, or making small changes to the Zotero translator).
We should also be able to significantly improve import performance further, but that may not happen for a while.