In 35:21, it says, "כל איש אשר נשאו לבו וכל אשר נדבה רוחו אותו הביאו"
There seems to be two different types of people donation to the Mishkan. There are those who are donating from their heart, and those from their spirit.
Those donating from their heart, the passuk says, "whose heart leads him" - it is an emotional donation. He is being overly generous, giving more than he otherwise would.
Those donating from their spirit are giving more in control - he wants to give but has to limit it.
The passuk equates these two types of people - It is not important how much one gives, but how one gives. The person who can allow himself to donate with no limit, just because he wants to is great. If a person gives wholeheartedly, even if he can only give less, but he gives what he can and he really wants to, he and his donation are just as important as the guy who gives millions, and even better than the guy who gives begrudgingly.
ובלבד שיכוון לבו לשמיים
NOTE: After I thought of this, I found the first part, describing the two different types of people donating explained on the Ohr HaChaim, much more eloquently than I could explain it.