If iMessage history is important, I'd highly look into exporting it. I haven't used iOS in a long while, but I believe there are programs (hopefully open source and trusted) that will let you convert your history to a readable format like HTML, PDF, etc that you can save securely somewhere. Also, keep a backup of the iMessage backups themselves (not just on iCloud) in case you ever want to use them again down the line.
Converting to Android will only bring over some history, but it will also break a lot of iMessage formatting. Things like group messages will not be brought over correctly or at all. For me, even using iSMS2Droid, messages from each person in the group went to their own individual threads (out of context) or even got assigned the wrong contact altogether. All of the random iMessage chat features also likely won't come through correctly at all.
Its also good security practice to limit how much sensitive data you store on your phone due to the potential for your phone to be lost, stolen, searched, or seized, so there are merits to not bringing it over.