Opinions please:

I am looking to replace an old Ideal Mexico boiler on an open vented system.
I have basically decided to go for a condensing boiler and like the look of
the Worcester-Bosch Greenstar 29HE or the Keston Celsius 25.

The main difference I can see between them is that the Keston has a
stainless steel heat exchanger, while the Worcester has aluminium / silicon.

Which of these is likely to have the longest life? (A couple of heating
engineers have said to me that the a condensing boiler will have a shorter
life than a traditional boiler as the condensate gradually corrodes the heat
exchanger. Thus my question as to which material is best.)

Also are there any other factors which would make one a better choice than
the other?

Any comments would be appreciated.
Thanks,
Garry.