Unless you have some pretty long lenses, you might want the extra resolution of the 50D to be able to crop your images, too. It's not just about enlargement. A 1.6X 15MP sensor is going to give you more fine detail than an 8MP 1.3X sensor can, which allows for more cropping, not just for bigger enlargements.
Personally I don't see all that much difference between 6.5fps and 8.5fps... But I try to avoid high frame rates most of the time anyway, since I hate sitting at the computer for hours and hours editing all the extra images a high frame rate tends to produce.
1D series has higher durability, a higher rated shutter life expectancy, better environmental sealing, all of which might be important.
OTOH, a six year old, used 1D MkII that was in the hands of a pro might have a couple hundred thousand clicks on it, while a one or two year old 50D that was owned by a little old grandmother might have only a few thousand. Last time I checked, shutter replacement was around $600 for a 1D and approx $300 for an xxD series camera. Batteries for a 1D series are, what, around $100 each? You can get generic BP511A off eBay for $6 or $8 apiece... Or perhaps better ones such as Sterlingtek for about $25.
Used simply, such as just the center AF point manually selected for focusing, the 50D is quite capable as a sports camera. It acquires quickly and tracks well in AI Srvo.... especially in good lighting such as you'd have most of the time when shooting surfing. Certainly, the 1D's AF would be better, and it will have some additional, more specialized AF capabilities that the 50D lacks. I managed about 95% acceptible AF accuracy when I was using three 50Ds, some of it in pretty difficult lighting. I imagine a good 1D, used right, might get you to 97 or 98% accuracy.
I really don't know when the 1D series went from 12 bit to 14 bit. It was the 30D/8MP to 40D/10MP upgrade that saw it in the xxD series.... Is the 8MP 1D MkII 14 bit, too? I don't know, but 14 bit also helps with image quality, cropping and enlargement. It's more subtle than sheer differences in resolution, but something you might want to consider and check on.
You'll also have to consider price... If they are different. A less expensive camera might leave you more in the bank to put towards a teleconverter or other things you need.
Of course, you should expect when asking this question here on POTN.... All the 50D users will step up to defend their purchase decision and all the 1D users will be quick to tell you why their particular camera is your only possible choice!
Very few of us actually use all the models and can give you head to head comparisons from personal experience. I know I don't and can't.... It's been years, and only a brief rental, since I used a 1D series... and that was the full frame model. I did use the old EOS-3 a lot for several years, which have similar AF system to the later 1V and 1D series. Actually the EOS-3 was the first cam to use the 45 point AF system (altho the it differed in that it had "Eye Control" - which I never could get to work for me - which the 1 series cameras didn't have.)
I can tell you I was happy using 50D for sports/action photography... and that I'm happy using 7D now. And I saw significant improvement in the "cropability" of images going from 8MP, 12 bit 30D to 15MP, 14 bit 50D. And I still would rather put my money in lenses, than camera bodies.
But at the prices you are looking at, you could almost buy either one, try it out for a while, then resell it at very little loss if you change your mind.