When looking for scanners and comparing them to a service there are a few things to consider.
1) quality. Some scanners will scan better than others. Usually you can tell the difference in the specs. A high dpi count with a good color gambit will usually yield a good result. Most prints are made (or recommended) at 300 dpi. So if you wanted to print your pics at 300 dpi, but you only had a 600 dpi scanner, you'd be able to print at twice the size of your negative. That's not even a 4x6. 2400 dpi or larger is what your goal is, but often times you wont need that much. For web purposes you can only really display at around 72 dpi, so a 600 dpi scan would let you enlarge it pretty big on the screen before distortion. Note that interpolated dpi is no where near as good at native dpi (sometimes called native resolution). Interpolation just means that the computer guesses at the extra pixels. The colors will be pretty close but if your looking for sharpness you probably want a higher native dpi.
2) time. Look at how long you wouldn't mind scanning your collection. If you want this done pretty soon, send it out. Many home scanners are not fast. I have an older Microtek and it takes about 65 seconds to scan 12 negatives (one pass). We used to have a scanner that only did one neg at a time, and that took 30 seconds. But then you also have to add the time it takes to set up the next slide/negative. It gets time consuming.
4) cost. If you want high quality it's going to take some cash. If your just doing a couple of negs at high quality you might just send it out. It'd be cheaper than buying a $2,000 machine. On the other hand if your looking at thousands of print quality negs then buying a home scanner may be what you would need to do. You could always get a cheap scanner for the "just for web" stuff, and send out those few gems that you want in high quality.
5) software. Your going to need something that will interface with your scanner and give you a proper color reading. All print quality scans will need to be sharpened. Most scans will need some tweaking. If this isn't something you'd want to do then you may want to send the work out.
Questions to ask when sending work out.
What will the dpi be?
What format will it be?
How will I receive my digital images?
What color gambit will it be? (sRGB is fine for most applications, Adobe RGB is better if your going to be editing the photos, like in photoshop)
How long will it take?
Are you insured against damage to my slides/negs?
Do you have a guarantee?
I hope this helps.
I have a bunch of regular 35mm print pictures that I would like to transfer to digital to put online, email, etc. What would be the best way to do this? I've seen advertisements for local places that say they can do this by using the actual print, or by using the negative. My question is, which way would give the best image quality? Or doesn't it matter? Please help. Any advice would be very appreciated:)