Reflective coatings (e glass) are now more or less universal as it’s a BC requirement for all replacement windows (except for conservatory's which don't come under BC yet) so you must ensure any replacement windows conform.
Argon gas is supposedly an additional benefit, I have it on all my new windows, both house & conservatory, but I’m not sure how much difference it makes in practice; I’m also unsure how long it will actually stay in there! If you've got the choice at no additional cost (as in my case) go for both.
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