I'm not an electrician, but I think it goes something like this. 100 amp service is pretty small, but it all depends on how much other stuff you have using electric and how well your box is balanced. By that, I mean that there are two 110 volt legs in your main box. Each leg can handle 50 amps in your case. 220 volt appliances are split evenly across the two legs. If your hot tub needs 50 amps, 25 amps will come from each leg. Your oven may draw 50 amp or 25 amps on each leg. If you have an electic water heater at about 60 amps, AC at 30 amps, and furnace at 40 amps, that would be 25+25+30+15+20 or 115 amps on each leg. That would be too much for 100 amp service. But you may not have all of that and all appliances don't run at the same time or at full capacity. 50 amp seems like a lot for a one pump tub and you won't likely have your oven running full blast. If you calculate the amps drawn by everything you have, it may exceed the amperage rating of your box. Your electrician should know the percentage to discount usage based on the assumption that not everything may run at once. You could also alternatively calulate the amperage of actual appliances and bulbs and determine your own likelihood of what might run at the same time. At any rate, this may give you enough knowledge to ask the electrician to explain in detail the load calculation for each leg to be sure you are comfortable that you need to increase service. If you agree that you need to increase service, it probably makes sense to go with 200 amp over 150 amp. I'm not an expert by any means and not even sure the above is acurate. Just my 2 cents worth. Maybe an electrician could jump in to advise before yours shows up. I'd probably go for the service upgrade to 200 amps.
