I think the Tesco site is probably easier but I shop online Asda as they are cheaper for what I buy (good basics and fruit and veg including smartprice) and my local one gives better delivery and service.Substitutions - always substitute for a more expensive product and don't charge you the difference.When something has been damaged ,near use by date they always give a refund and have in this instance sometimes redelivered the product later that day without a charge.They did this Christmas Eve for us from our delivery the day before.
Tesco were iffy on service as were Ocado in my area but I think this very much depends on your local delivery.
With all the 1st shop online takes a while but you can save a shopping list and it is pretty easy next time round.I would try both a couple of times in your area and see who gives the best service.