Upholstered chairs are often more comfortable and usually look lavish, but keeping them clean and looking their best may involve more work that you are expecting. When you're in a room that will naturally have spills and messes, upholstery can be a problem.
If you decide to go for upholstered chairs, synthetic fabrics designed to resist spills and staining (think ultrasuede or polyester and viscose blends) are a good choice, unlike velvet, velour or chenilles, which are too demanding for a dining area. Leather provides comfort and durability too, but it tends to be on the expensive side of things and requires special maintanance. No matter the material, always consider Scotchgard protection to lengthen the life of your upholstery.
Slipcovers can help you protect your upholstery. You can enjoy the comfort of a thick, upholstered chair but not worry if someone spills their wine at dinner. The slipcover can be removed and washed. However, then you are not enjoying the beauty of the chair itself, so it can be a bit of a trade off.
If you decide to utilize slipcovers, you'll need to measure the seat and back of your chairs. Most slipcovers require these measurements. Choose a material for the slipcover that is machine washable so that washing is a breeze.