The best way to address this question would be to think about how the deck will be used.
If it’s likely that wheelchairs, prams or pushchairs will be used on the deck then smooth boards would be preferable as they would simply be more comfortable for the user.
For anyone wearing heeled shoes, smooth boards would also be a better choice as grooved decks can be much more tricky to navigate. This also heightens the risk of damage to the boards, not to mention the shoes!
If the deck is often used for barbecues or outdoor eating then food debris can become trapped in grooves. A smooth deck will be easier to clean, so perhaps a better choice in such circumstances.
There is a school of thought that suggests grooved boards offer better anti-slip properties. In practice this is not the case.
Why choose a grooved deck then? One very good reason is appearance. Everyone will have their own opinion on what looks good of course, but many people feel that a grooved deck is perhaps aesthetically more pleasing. It’s also perfectly possible to combine smooth and grooved decking boards to personalise the design.
A major advantage of grooved decking is that it can aid water run-off. This is true - up to a point. The deck would need to be installed with a slight fall and the boards would need to be less than 3.6 metres in length. On a longer board water surface tension comes into play and there is a good chance that the deck will then hold water rather than dispel it. If the deck boards are to be longer than 3.6 metres in length, then smooth would be the better choice.
Something else to consider is that when treating the deck with an oil or stain, a grooved surface will usually require more product.
Once you’ve decided what type of deck to install it’s worth giving some thought on the best time of year to have this done. All decks will need some sort of treatment, such as Decking Oil, Decking Stain or Decking Paint.
Before these treatments can be used a new deck needs a period of weathering to allow the grain to open up and natural moisture to evaporate away. This period can be anywhere from 6-8 weeks for a softwood to maybe a year for very tightly grained and oily hardwoods, such as Iroko. Exotic hardwoods tend to need a thinner consistency oil, such as Osmo 007 Clear, Osmo 019 Grey or Osmo 020 Black.
New decking boards often come with a basic water proofing pre-treatment. This can be checked by dropping a little water on the deck. If it beads on the surface then a pre-treatment has been applied. If this is the case it may take 6 months for the pre-treatment to wear off and until then the deck would not accept a stain or oil. However, Ronseal Decking Cleaner and Reviver can remove these coatings and therefore shorten the time before a new deck can be treated. It may take more than one go, so it's best to do a water droplet test after applying this product. Once the water no longer beads on the surface then the pre-treatment will be gone.
Decking products need to be applied in dry conditions and usually cannot be applied if the ambient temperature is below 10 degrees centigrade. Consequently there is a ‘decking season’ that runs throughout the warmer, dryer months. It would be a good idea to time the installation of the deck, so that it has sufficiently weathered by late spring, by which time the weather has (theoretically!) become more reliable.
If the wood has ‘silvered’ off during the weathering process, then this can be reversed by using a product such as Barrettine Wood Reviver. This will restore the wood to it’s original colour prior to treatment.