Buy the one, which is cheapest, having used OE, and pattern, the only difference is price.
As soon as I find a part that falls within my mental venn diagram for convenience(including finding and acquiring), price and quality I buy it and move on. I can't imagine buying a common and cheap part like this from America, when I can get it from any local motor factor and examine the thing before I pay for it. Or order it from ebay at 19:57 on Sunday and fit it when I get home on Tuesday - total time sourcing the pump might run to a couple of minutes.
Ironically it's the parts that were always GM only that cause the most problems, as GM(any manufacturer) don't care about cars that are at least twenty years old, and have been devaluing the stock and getting rid of it. So if you think you're going to need a
self reciprocating side fumbling dingle arm at some point, it's probably worth getting one now. I wouldn't be surprised if the normal aftermarket service parts - pumps, plugs, filters, belts, pullies, wishbones etc - outnumber the remaining cars by a large margin. That's how the classic parts specialists can still supply NOS for 60 year old cars, although rubber parts need to be carefully inspected for condition before buying.