Semco is a good product, available in many 'hues'. However like most sealers it can eventually 'transfer' to the bottom of your deck shoes, making for poor traction.
Havent used Teak Guard, but it looks like and is reported to be very good.
Another Product I use is Teak Wonder - a clear sealer.
My 'usual' sealer mix for teak decks and 'bare' trim is a mixture of: 1/3 Semco 'goldtone', 1/3 Teak wonder, 1/3 Thompsons water seal. This is the best mix Ive ever found to withstand the rigors of 'green water over the bow'. Ive found that such a mix will last for about 6+ months when 'double coated'; and, if the coating begins show-through the teak then a quick 'diluted spit coat' will extend the life by another 4-6 months. Ive found that the above 'mix' will adequately protect the teak, still allows the teak to 'wet out' thus maintaining just slightly less 'wet traction' than fresh bare teak. I usually etch all my sealed teak every two years with TSP followed up by bleaching with oxalic acid --- my sealed teak always looks 'freshly sawn / sanded'.
Ive found that 'green salt water over the bow' will greatly shorten the service life of most teak sealers.
My cruising boat is a 'teakey' has full teak decks and cockpit; some sisterships use the above 'mix' for ALL their teak with quite outstanding and long lasting results. With the current price of teak straking, etc., its really prudent to protect teak decks, etc.
You really dont want cetol, varnish, etc. on those surface that are 'walked on' or 'sat upon' --- mega slippery when wet
that stuff tastes funny