This seems unlikely to be a useful tie-break in most cases, provided one can switch membership. UK party leadership elections are rarely contemporaneous  (unlike in the US), so the likelihood of a given party member being able to realise their leverage will generally differ by more than a factor of 4.5x at any given time.
 Conservatives: 1975, 1990, 1997, 2001, 2005, 2019
Labour: 1980, 1983, 1992, 1994, 2010, 2015, 2016, 2020
Hmm, but is it good or sustainable to repeatedly switch parties?
Sorry I should have disclaimed that I don’t think this is a sensible strategy, and that people should approach party membership in good faith (for roughly the reasons Greg outlines above). Thanks for prompting me to clarify this.
My comment was just to point out that timing is an important factor in leverage-per-member.