Sounds like more than 1 problem the exp vessel is defo flat and will just be a matter of time before its coming out the prp, it's also cycling so that could be a water quality issue poss a chocked return filter and the heat isn't leaving the boiler fast enough, have you been adding a lot of...
The difference is really more to do with hot water output and boiler range, Worcester have three ranges the junior is the cheapest boiler the si is middle of the road and the CDI is top of the line, if you use the bath a lot I would recommend going for the 30si if you could afford it although I...