Are there any other obvious alternatives we should consider, or does
anyone have experience of the above two solutions?

If there's a mains feed, you could consider a venturi shower.

However, provided it is of moderate flow rate, there's nothing to stop you
installing a pump. With high flow rates, you might get issues sucking the
vent dry unless you install a flange (and possibly additional pipework).
However, you should get away without doing this invasive work provided that
the flow rate is limited and the route from the cold tank to the hot
cylinder is reasonably good (at least 22mm with few bends and wide open
valves).

Christiasn.