There are typically 3 weights of T/O blankets (light weight, mid-weight & heavy, with the amount of fiberfill and the price increasing as the weight goes up. There are also T/O sheets which have no fiberfill. T/O’s are designed to keep the horse dry when turned out, but can also be used as stall blankets, but stall blankets are not waterproof, and cannot be waterproofed because even the nylon ones do not have a membrane fused to the underside of the nylon material. This membrane + the chemical coating on the top side are what make the T/O waterproof. The true enemy of the waterproofing on T/O’s is UVA light, which destroys both the top chemical coating and the fused-on membrane. The top coating can be replaced, but when the membrane starts to defuse, you basically have a stall blanket. Horses that stay dry have a better chance of staying warm, but too heavy a blanket can make them sweat, so they are damp, and can get cold. The best answer I have found is a T/O sheet for early spring and fall, + a mid-weight T/O blanket for most of mid-winter, plus a blanket liner for the really cold days. If it gets too cold for just the sheet, but not cold enough for the mid-weight, you can put the blanket liner on under the sheet. If the horse rips the blanket mid-season, the sheet/blanket liner combination can be used for a short period while the blanket is being repaired. This can be important as blankets have to be washed & dried to be worked on, and the longer you leave the damaged blanket on, the more damage there will be. What was an easy repair one night could be a blanket shredded beyond repair 24 hours later. The clip you choose will probably be dictated by how sweated up your horse gets in a normal winter work session. Some horses sweat more than others, and the goal is usually to remove the least amount of hair so that the horse can be cooled out quickly. A polar fleece or wool cooler is also helpful to put on the horse while he/she is drying, as you do not want to put the blanket on a wet horse – – the dampness will wick into the blanket and the horse will be damp until his/her body heat dries the lining. Polar fleece & wool will both pull the water out of the horses coat into the cooler, but polar fleece can be machine washed and dried. You will need to plan time for the cooler to wick the water out into your ride schedule, unless you have some ride buddies who meet to ride at the same time, and you can rotate who stays behind to pull off the coolers, hang them on racks to dry, and put blankets back on.
Pay attention to the D-rating on the blanket, which is a measure of the sturdiness of the material. Anything less than 1200-D will not hold up under normal wear and tear, unless your horse is turned out alone, and maybe not then. Horses in a group T/O area will play blanket pull, and that is the source of at least 75% of the damage done to blankets, and explains why so much of the damage is in the rump area, which is hard for the horse wearing the blanket to reach.
Hope this info is helful.