Both are extremely good tools. Like you I don't have a drill press, however in my case I opt to get my blocks from John instead.
I think it would be fair to say that the gold standard would be the block/silver bullet... no matter how precisely square the wood is (or perhaps unprecisely is the better term), the block will allow you to drill your rear axel holes in perfect relation to one another irrespective of the wood....
I used Ian's tool like it was going out of business for my pack race....there were kids and parents in line for me to drill a hole into the precut slots and it made everyones life much much easier!!!! (MUCH THANKS IAN FOR THE TOOL!) However I did notice a few issues....for it to drill both rear axels nice and straight in relation to one another, it has to fit nice and snug on the block and your at least partially relying on the block itself to be decently cut and square....and if the kids already started sanding the block or shaping it then it can be difficult to either get it fit snug or fit so the holes are exactly perpendicular to the body. Even when it fit well, I had to clamp it down hard to prevent it or the block from moving around when I was drilling.
But I think these are all very minor issues that I knew going in and fact is, even with that its still 10x better than using the slots by themselves or handrilling without it or whatever people were doing before....A bunch of people were asking where I bought it (particularly since it has both straight drill holes and canted drill holes!)
Maybe Ian could comment on some suggestions for those cars that don't quite fit snug or ideas on some of the stuff I mentioned....I was putting tape on the sides or a papertowel to make it snug at times....clamps are fine but some people wanted me to drill the holes and the car was already painted....and im like damn I don't want to dent the paint and body with a clamp!
But my solution is next year, im going to make it a point to say that if people want their block and slots drilled out then I need to do it before they start shaping the car!!! (I know, kinda common sense) I think that should take care of most of the concerns I saw this year.
But if you want Ian's one better order fast LOL...I haven't checked recently but there was only a small supply left!
WK