I think you will find many reasons for many folks casting their votes in many different ways. You are getting into horse race country here. I cast 29,000, one-dollar votes for a TMX-IO-360 vertical draft with a Hartzel.
Consider all the issues and make your own dollar-conscious decision:
Constant speed or fixed pitch - weight and cost as opposed to performance.
IO or O - economy when LOP and no carb ice versus cost and complexity and weight.
180, 200 or 210 hp - a little bit of difference in cruise but not much, improvement in vertical performance in exchange for big cost and weight difference.
Vertical or horizontal draft (IO consideration) - aesthetics (to each his own but I like the looks of the vertical draft cowl) and cost versus a bit more power.
In the end, you can go with a new O-360, fixed pitch and a Sensenich for around $23,400 or a 200 HP IO-360 with a Hartzel for $41,500 (based on today's prices from a major builder's site). This is a stark demonstration of the price differences the decision entails.
For me, buying a vertical draft IO-360 with a Hartzel was the best performance/cost equation for my mission. Cruise, climb and takeoff distance got their biggest improvements by spending the extra $4000 on the CS prop. The extra ponies would have cost many extra Benjamin's and not provided the performance per dollar as did the CS prop.
Like I said, you're entering the area of opinions. Buy what you can afford and meets your needs.
As per the EI, P-mag seems to be a great product that is small, light weight, unobtrusive, reliable and fully self powered at over 700-800 rpm. E-mag has the above qualities except for the self-powered feature. Using one of each seems to provide an inexpensive means for installing EI that works in all regimes and will continue working without outside power. I bought 1 of each on my TMX for an additional cost of $705 after the credits for the standard ignition. My untrained opinion is that the main reason for retaining 1 standard mag is to eliminate the risk that an electrical failure could cause loss of power. This is fixed by installing 1 P-mag, so in for an ounce, in for a pound.
Jekyll