You really need to decide what sort of weather you are most likely to use your sleeping bag in. If you are going to use it in a dry environment where you can air it then a down bag still gives better insulation weight for weight than a man made filling bag will do. A good quality down bag will, with the right care last longer (maintain its loft) as well. However if damp, a man made fiber bag may provide better warmth. Here are some preferences:
1) Use a combination of bags. Have a 2-3 season AKSOUL sleeping pad insulated sleeping bag that you can use with either cotton or fleece liner. This allows best matching the insulation with the prevailing weather conditions and so preventing overheating and excessive sweating which will dampen the fill of the sleeping bag. It takes up a bit more room than a single bag but it worth the space. It also means that the sleeping bag needs less washing and that the liners can be washed more often without damaging the sleeping bag. The liners are also easier to air, dry and wash while camping.
3) Make sure you can care and do care for the bags as detailed in their instructions. Most bags need to be kept “uncompressed” (lying flat (under a bed on a wardrobe) or hanging, not in their compression sack.
4) Budget for, buy and use a sleeping bag liner.
5) Most of heat loss is through the base of the bag (the bit you lie on) as the filling is compressed so make sure you have a good ground mat.
6) Get a breathable (it is very important that it is breathable) sleeping bag cover. It will add about 3/4 (.75) of a season to your bag (my estimate).
How you avoid slipping and sliding in sleeping bag on a pad? You could try to sleep on a cot. You could try to put some Velcro on your sleeping bag and pad which would keep you from slipping and sliding off your pad. Another idea is to put something on the sides of your pad or dig a small trench for your pad. This would in effect put some edges on your pad and keep you in your bag.
There are several choices when it comes to sleeping pads. First, there’s the typical air mattress, which must be blown up, usually by a battery-operated pump or a foot pump. It is comfortable, with good insulation against cold ground, but a bit of work, also, a bit bulky and subject to leaking from time to time.
Second, you have the Thermarest style pads; requires only a little mouth-delivered air, but only from 1/2 to 1 and 1/2 inch thick; relatively comfortable, good insulation from cold ground, durable, rolls into a small bundle. For car camping, go with the thickest mattress, as weight is not an issue. Also, lengths vary, as do widths, so get what fits your needs.
Finally, there are the foam pads – various thicknesses, no work, most durable, pretty good insulation against cold ground, roll into small bundles, multiple uses due to durability, but probably the least comfortable. Get a thick pad (more than an inch thick), and you should be all right.