Pixiq

It depends on what kind of scanner you have access to, and where you would take them to be scanned if you were to use a processing service. Dedicated film scanners tend to do a much better job than flatbed scanners, but they also cost a lot more.

The Nikon COOLSCAN 9000 ED would give you totally professional results, but with an MSRP around $2200, it might make more sense to pay your local pro lab a few hundred dollars to do it for you, depending on how many slides and negatives you need to digitize.

I have had very good luck with an earlier version of the Epson V700, which is considerably less expensive, plus is equipped to scan large format negatives and slides up to 8x10 (the Nikon only does 35mm, 120, and 220).

At any rate, if you do decide to go with a processing service, I recommend having them process just a few slides and negatives at first to see if you like what they do. Give them a variety: at least one of each format if you have different formats, some black and white, something that's heavy on the greens (like your Velvia slides), another one with primarily warm colors (your Agfa negatives), one with high contrast, one with low contrast. When you get the files back, look at the images carefully. Check the colors and the contrast to make sure it's all accurately rendered. A good lab will charge you a pretty penny to scan them all for you, but you'll likely be happy with the quality, and think of the hours and hours you won't spend painstakingly cleaning and scanning all that film!