As the saying goes - you can't get there from here (51SE to REX), hence the log jam of 51SE's for sale. The upgrade path ended at the 51SE. Most will need to start or 'upgrade' with/to the 52SE, then add the REX upgrade, as funds allow, unless pockets are quite deep.
The 52SE has a second generation Super-Pak increasing capacitance even further over the 51SE, adding further "textural liquidity and coherence" as the BAT website states. An improvement, yes, however the 52SE seems to have been created to be the launching pad toward the REX, BAT's statement piece. Unless you plan on eventually going for the REX, your current 51SE may suit you fine for quite some time, at least until used 52SE's start showing up for sale.