(I’m sorry this turned out to be long winded, and only the first part really answers your question, the rest is early morning ramblings)
The SV650 is considered forgiving because:
1- The engine is very smooth in its power devilery. Down low there is a lot of torque (so you don’t have to shift as much), and as the rpms rise, so does the power. This is different from a “peaky” inline four where there is almost no low down power (requiring a lot of clutch feathering in traffic) and very little power below a certain rpm, however once you pass that rpm the power rises very very quickly. This extra power delivery is an issue because if you hit a bump and your wrist twists you’ll unexpectedly be accelerated forward. Especially if you are cruising right near this transition zone – you could mean to accelerate a little bit and find yourself doing muich more.
2- The bike is responsive, without being twitchy. A supersport does exactly what you tell it. Even if you just hit a bump and that “Telling” was your body recovering from the bump. New riders (just like new drivers) tend to over correct mistakes, which make matters worse. The SV650 is supposed to be pretty good about not reacting as hard to unintended actions. If you press too hard with your hand, it’ll still turn, but it won’t whip around like a supersport might.
3- The ergonimics are comfortable. This is a big one that you don’t hear as often. When you are on an uncomfortable bike, you get tired faster, which leads to mistakes.
In my area the SV650 is a favourite. A lot of the local guys have them. Most who do have been riding bikes for years, yet they recommend them to new riders who for whatever reason will have nothing to do with a 250.
That said, your comment about being in for the long haul might point you towards the 250. If your goal is not not resell your bike, the the 250 may or may not be for you (some people keep theirs for a very long time).
But if your goal is to be in motorcycling for a long time, the 250 is probably the better choice. Why? Because it teaches you things the bigger SV650 won’t (or rather, the SV won’t teach them as quickly). It is even more forgiving in the corners, and you can ride it near its limits in legal settings (after a few years of riding experience). The added skills and confidence of knowing how to ride corners hard will make you a much better rider (as opposed to the “shoot down the straights, slow down for every corner, then accelerate like a mad man again” approach).
Both are excellent bikes, and you’d enjoy both. But, most everywhere else in the world uses a graduated licensing system capping learners on 125cc machines, and sometimes having another cap after that before a full license. In these countries, motorcycles are daily transportation for much larger percent of the population than in North America. There are good reasons for these caps, and many people (notably ex-pat brits around here) feel these systesm did them a world of good.
Look a the reasons you prefer the SV over the 250R. (“I like the look better”, “I like the thought of having more power”, “I think I’ll have more fun on the 650”). Some of these really won’t matter in the long run (the latter two for sure, the former maybe). Others, like (“I find the 250 uncomfortable”) are very valid reason.
When it comes down to it, you are spending a great deal of money on something. I feel you have to really love that something to spend that much money on it. If the 250 just doesn’t invoke any feelings of lust, and it doesn’t put a smile on your face, then you have to look elsewhere. If the 250 does, but the 650 seems like a better deal because it has more power and more growth potention, take another look at the 250, it may be the bike for you for.